If Ricimer died/was killed, and Majorian was succeeded by a strong leader, why couldn't the Romans play off the foederati against each other to consolidate their power?
The problem isn't about leaders, it's about resources and particularly military resources : Majorian's army was largely made of foederati, even if he tried to deepen the use of local militia, and it's why his campaigns (against Visigoths, for instance) had as objective not to crush their power but to integrate them into his own armies.
Integrating them doesn't mean, at the latest, destroy politicization and power influence in the armies : Ricimer's influnce over Italian issued armies can point that.
Assuming Ricimer dies, his political network isn't going anywhere : he's likely to be succeed either by a nepos as Gondovald, or another important Romano-Barbarian figure.
Basically, late WRE Emperors had to face military resources issues (under which you always found fiscal issues) and political factionalisation : Ricimer isn't just a random Barbarian who happened to be the grey eminence of his time, but someone benefiting from real influence and support among foederati and Romano-Barbarian armies as well than in the senatorial aristocracy.
One could even say that if he died and not replaced, WRE would probably have crumbled even more quickly into political anarchy, as it happen after his death.
What do you think the WRE would be like if it was slowly absorbed by Constantinople?
Mostly because WRE would have been either more and more reliant on foederati and Romano-Barbarian armies to the point being absorbated by these as IOTL, or either it becomes reliant on Eastern Romania's resources and at best becomes a mega-exarchate.
Would the ERE be able to hold onto those territories more sustainably than in Justinian's wars?
I think so : Justinian wars, especially in Italy, had a relatively flawed political vision of the post-Imperial west. Gothic Wars lasted for decades, and in Africa even with Vandals quickly defeated Byzantines had to fight (and loose territorialy) against Mauri.
ITTL, not only the Romano-Barbarian structures doesn't absorb WRE (meaning a lesser structural resistance), but Constantinople would likely have a more accurate political, and therefore strategical, vision of the problem.
Doesn't mean they will hold because of this, but IMO, it does give them a serious advantage : shorter wars, no restructuring effort, etc.
Would they become Greek eventually?
ERE wasn't much as Greek than mostly (and both) Hellenic and Latin : let's not forget that Romanisation, in the East, was made along Hellenic features since the Ist century.
This romanisation included, by the Late Empire, what one would call "structural romanisation", channeled trough roman religious, cultural and social features and a more "creolized romanisation" that touched more deeply the overall population but formed distinct identities, cultures, etc.
On several grounds, it's fair to point that western Romana could be less romanised than its eastern equivalent. Of course, regions as Italy or Africa, even with the fall of WRE, were following a same pattern than ERE*, but long story short, an imperial unification (let's remember that WRE and ERE are historiographical concepts, and that people then only saw a bicephal but same Roman state) from Constantinople would likely not turn into an Hellenisation of Italy.
*Roughly speaking, one can distinguish a post-imperial Romanity which is distinguished by episcopalian cities, roman law, and Latin as common language; and late imperial Romanity with municipal cities, maintain of classical spectacles and classical schools (even if all of these can be found elsewhere, but not distinctively).
Was the Vandal hold on Africa that strong?
It was. Again, we're talking military and fiscal resources there and Vandals had both.
Battle of Cape Bon asked for a really strong ERE support (even if the whole 100 000 men and more than a thousand of ships can be debatable), and even that failed before Vandal navy.
Justinian had to wait for Vandals having their asses kicked by a general Mauri advance in Africa to crush them.
I stress the naval projection part : WRE simply didn't have one that could match with ERE or Vandals. When Majorian fleet is destroyed, he simply can't pull another one out of his sleeves (and this failure may explain, partially, his murder).
Now could it have worked? It could have, not without an harsh fight, but it's well into the realm of possibility : it wouldn't have pointed anything, however than the great dependence of Ravenna from ERE's resources. The whole cost of the expedition largely exceeded WRE fiscal revenues, representing more than one year of what ERE could gather itself.
And, eventually, it's not about Majorian or Ricimer themselves, but about the structures they used and represented. Even with Ricimer alive, the network of political factionalism and alliance would still be there, both in Ravenna and Constantinople.
Now, a reconquest of Africa before 468 could likely have been attempted more successfully, my point isn't that it was bound to fail militarily.
We could as well assume that Marcellinus succeed Majorian, Ricimer failing for some reason to evict the general out of Italy or having more interests dealing against Vandals, and conquers Africa.
Or that, somehow, Majorian's fleet isn't destroyed and manages to reach African coast victoriously.
We'd have then a Roman (meaning militiae, regulars, Barbarians, foederati) army in Africa, and have to deal with Vandals, after what was probably an harsh naval fight, and probably going for an harsh battle. But again, Romans could take the best of it and win.
What happens? Well, more the emperor is out if Italy, more Ricimer's power (or his successor, or any magister militium wannabe) is increasing and more chance seeing allies and foederati going their own way.
When Majorian/Marcellinus/etc. would have leaved Africa :
- Either the region is militarily supported trough ERE's resources (and at this point, Constantinople is more than wary about western figures) meaning a ERE absorbs WRE provinces
- Or you'd have Vandals, still in Africa, probably reduced to their original foedus in Mauretania (remember, Majorian/Marcellinus would have needed men and can't afford, or even can, crush Romano-Barbarian entities) taking back most of it , as Visigoths did with Gaul and Spain IOTL.
Except, this time, they would probably struggle more with Berbers or other players.
And I don't go into the problems of usurpers : late WRE had plenty of it, and any ambitious general willing could have seen in Africa a fair base, as it existed IOTL in Dalmatia, Gaul, etc.
To me, the problem isn't how Romans could have won in Africa, but how could have the WRE managed to win and hold it at this point.