With that in mind,is it fair to say that if a Pagan like Marcellinus was to get rid of Ricimer and proclaim himself emperor,the Eastern court would have just ignored the prospect of a pagan restoration?
It's not really clear if Marcellinus was pagan or not.You might say it's a bit irrelevant, giving that it's a convenient accusation as it was with Anthemius.
As for Constantinople, Marcellinus could pull a Julius Nepos without real problem, as long he gets Constantinople's agreement.
Italy is the empire, but Italy alone has not the resources available to Marcellinus, Aegidius and Nepotianus.
Which is irrelevant, tough : there's no real way that the various Gallo-Romans leaders and Dalmatia could poll their ressources all together.. Italy, for all the crisis and issues it had, srill had important resources comparatively and, which is significant, enough legitimacy to recieve support from immediate neighbours and foederati which none of the aformentioned provicial rulers really could (while still depending more or less largely from Barbarian auxiliaries/mercenaries whom loyalty wouldn't be obvious).
Surely they need to deal with local issues, Nepotianus should be busy with the Suebis while Aegidius need to deal with Theodoricus II but this is only a couple of years after Majorian’s campaign in Gaul and Spain.
Agreement with Barbarian petty-kings and the empire was mostly considered by the former as personal agreements : meaning that by the death of Majorian they considered their position at best as open to renegociation, if not allowed to enforce directly their own interests.
Note that provincial roman elites, especially in Gaul and Spain, were more than open themselves to negociate directly with Barbarians, due to a lack of sense of loyalty or legitimacy with Ravenna's : it's basically how Burgondians managed to go this much south IOTL.
We should also consider that in addition to the local armies (doesn’t matter here barbarian or Roman) there was still the army that was supposed to invade Africa with the emperor.
This army was quickly disbanded, and was mostlymade up of foederati and mercenaries. That they wouldn't consider themselves tied to the Empire for the time being, and more close to their own direct rulers is decisive into understanding Barbarian politics in the Vth, and why foederati managed to get back territories they lost during Majorian's reign so quickly. egidius' exemple is particularily interesting in that he couldn't count either on foederati, either on a significant part of Gallo-Roman elite, his rule in Northern-Gaul being more of a sanctuarisation of territory than anything, one he had to fight for constantly since 461. You should not underestimate the network of conflicting loyalties in provinces that generally played against Imperial unity from one hand, or even regional unity (Auvergne, while a "national redoubt" of Romania in Gaul never acknwowledged Aegidius)
It wouldn’t be impossible to see one of the three generals hire again these troops ( as Aegidius did with some of these troops).
They wouldn't have the resources for that : Aegidius raised Barbarian mercenaries and made alliance with Franks and Britto-Romans, but these weren't the same troops raised for the African campaign in all likeness.
Furthermore the bribery of soldiers can work both way, there is no reason to assume that the Italian army would stay loyal to Ricimer until the end ( especially considering that his position as master of the empire is recent).
While improverished, Italy still had more resources than what remained of provincial authority : that Marcellinus couldn't copete with Ricimer's bribery temptatives of his army is, IMO, implying this just well enough. Note that Ricimer's authority isn't just based on these resources : his position might be recent but it's a strong one and sanctioned by the Senata, meaning it's still more legitimate for many than semi-roguish provincial rulers not acknowledged entirely by Constantinople. Ricimer's authority? It's as what foederati and most were concerned the imperial authority wheater it was respected or not. Marcellinus or Aegidius' authority? Not the imperial authority, and generally not respected and considered at best as on part with petty-kings'.
You could call this a legal fiction (altough, IMO, it's really not) but it helps making the difference at an institutional and political level.
Your proposal of a different Déols is interesting ( considering the misterious nature of Rhiotamus)
The most solid assumption on Riothamus IMO (which is probably a surname, giving it basically means high-king) is associating him with the historical Ambrosius Aurelianus.
but I would like to explore the possibility of one of Majorian’s general (or all of them) avenging his emperor
At this point, they basically have to save themselves against a general pressure due to Majorian's death : not just against Ricimer but basically every opportunist attack. You'd need first to ensure their political-military survival as they did IOTL before going on a rampage (assuming they can treat with foederati and Barbarians in the way)
But would the war really last so long (and devastating)?
Such an expedition would break a lot of the status-quo in western Romania : Ricimer demonstrated time and time again he was resourceful enough that Constantinople preferred to deal with him rather than "deal with him". Ricimer would benefit from senatorial and military support due to an outright invasion, plus alliances with relatively strong foederati : the Gothic Wars is a good example on how just throwing troops at Italy doesn't mean an easy, flowery and joyous campaign. Note that Ricimer may be killed/taken care of relatively quickly but the vaacum power wouldn't benefit Constantinople at this point, and having a Barbarian king as Gondioc appearing as the legitimate alternative against Constantinople literally trampling over Ravenna would certainly play a lot : at this point Byzzies would have a choice, either managing the situation which would mean maintaining troops and campaigning in north Italy and Gaul, either acknowledging what they probably realized immediatly IOTL that the whole situation was complex and that it was better to manage it via proxy. In the second choice, the whole expedition would be effectively more limited, but as I said above, only a costlier, more destructuring and wholly unefficient way to pull a Julius Nepos.
It was crystal clear for Leo that searching an agreement with Ricimer was war more efficient to ensure Imperial authority than overdosing on testosterone.
Especially when it was so common for the Romans to leave the losing side in favour of the winner.
That's really oversmplistic : what happened in the provinces does shows that conflicting loyalties and refusal of imperial authority played a major role in abandonment of whole regions to Barbarians.
I agree. Ricimer,while having a working relationship with the Italian elite,was far from being seen as legitimate
And yet, he was : having troubles doesn't mean being illegitimate (if it was no emperor in Roman history was legitimate giving the records of revolts or usurpation). His own support base was of course not particularily strong but it wasn't weak either. That it was based on institutional matters doesn't make it less true.
Again, we're talking of someone that made his way trough the troubles without major campaigns to assert his rule (one that he might have attempted in Gaul in the 460's essentially failed, without major repercussions). The idea that Ricimer was an opportunist Barbarian that only managed to get in charge because he murdered everyone while Italians were too lazy/frightened to act is...well, obsolete.
I think only a small force is required for long term maintenance,like 5,000 soldiers perhaps,to protect the Western Emperor until he can build up his own supporters.
Even Julius Nepos benefited from more than that. At some point, it should be clear that neither Ricimer and Italo-Romans, or Barbarians were push-overs. Frankly, if the solution was so obvious, don't you think it would have, I'm not saying attempted, but at least be considered IOTL?
The point of such a small force is only to remind potential opportunists that the ERE is ready to defend the Western Emperor and that they will react violently to a potential usurpation.
Or more likely they would merge with the general Barbarian mercenariate and support either a WRE usurper (regardless if they pulled him or not) with the commander acting on its own, especially as things got to change in Constantinople. Remember that such army would be essentially made up of Barbarians mercenaries and foederati.
As long as the ERE seems ready to back the Western Emperor militarily,it will change things a lot.
Indeed, it would make people frowning upon Constantinople's meddling mucb more polarized, and foederati seeing it as a likely threat, in the tune of "It could happen to you too", making Visigoths looking less threatening by comparison.