I would point out that maps in this age change very quickly and in most cases aren't worth a lot. The Byzantine power was concentrated in the coastal cities of Southern Apulia and Calabria.Northern Apulia and the interior were governed by Longobard lords, who from time to time professed nominal allegiance to the ERE, to react to the encroachments of the larger Longobard states (Benevento, Capua, Salerno, Spoleto). Gaeta, Naples and Amalfi were no longer under Byzantine suzerainety since the 9th-10th century (check the link I gave you)
Well, a map is easier than hunting down a sufficiently lengthy description from one of the books I have on the period, particularly since my book collection is disorganized as heck.
And not having Gaeta, Naples or Amalfi does not mean that all of Southern Italy is independent.
It looks like you are not going to consider different view points.
I'm fine with considering different viewpoints. I'm not fine with the unsupported idea that the emperor gains nothing from holding southern Italy and that the empire is incapable of doing so because that is not supported by the actual history of the Byzantine Empire.
The funny thing is that you agree that Maniakes was undermined by court politics, but you don't want to draw the conclusion: this would happen every time that a catepan would be in a position to succeed, because a successful catepan (even worse if he controls Sicily too) would have a power base far enough and large enough to make a bid for the throne.
From Norwich :
1) "But the collapse of the Byzantine forces after the victory at Syracuse was sudden and complete. The fault seems to have lain partly with Maniakes and partly with Stephen, for whom the general had never bothered to hide his contempt and upon whom, after some worse-than-usual ineptitude, he launched a violent attack. Stephen then accused him of treason. Maniakes was called back to the capital adn imprisoned..."
2) "Then, for the second time in two years, Georrge Maniakes fell victim to palace intrigue. His enemy on this occasion was the Sclerinia's brother Romanus. The two possessed adjoining Anatolian estates, and relations between them and been poisoned by territorial disputes. Now,finding himself a member of the Emperor's intimate circle, Romanus engineered Maniakes's recall. Meanwhile, profiting by the latter's absence, he looted his house, laid waste to his estate, and finally seduced his wife."
Maniakes being of either shaky loyalty or excessive ambition or even the fear of such are not the kind of politics that stabbed him in the back and undermined a perfectly feasible military campaign - twice.
So no, a successful catepan is not automatically going to be seen as a threat to the throne and undermined. Its a possibility, sure, but its not what undermined Maniakes, so you're going to have to find another example.
The other selective blindness is the failure to accept that the core lands of the ERE are the Balkans and Anatolia: Italy is not. The ERE is hard pressed enough to hold its own against the Slavs in the north and the Turks in the south. The last thing they need is to open a new front in Italy and pour resources into it.
No one is refusing to accept that the core lands of the ERE are the Balkans and Anatolia, but you are refusing to see any way that someone who can control the Italian lands can use them to
strengthen the empire's position and are treating it as if all it is either barren or rebellious to the point of independent in all but name no matter what.
In other words, Italy is
a source of resources as well as "a new front".
Now, if your argument is that until and unless the Balkans and Anatolia are secured than attempting to (re)take southern Italy is a distraction at best, then we're not in disagreement, but that's different than saying the area isn't worth holding.
Also, of course, the empire is not doomed to be struggling against the modest Slav threat and the temporary Turkish one forever.
A final question: did the acquisition of the southern kingdom do good for the Houenstaufen? Everyone would say yes: it was consensual, and they got a rich kingdom, stable and centralised. However the Houenstaufen ended badly: it may be controversial, but I am convinced that in the long run the HRE would have benefitted by not acquiring the Norman kingdom (and I am absolutely sure that the best solution for the people of southern Italy would have been to remain an independent kingdom, under its own dinasty). The same applies to the ERE too.
They ended badly despite, not because of, acquiring (the kingdom of) Sicily. Frederick II focusing too much on Sicily was a problem, but focusing too much on any area at the expense of the empire on the whole would have been equally unhealthy.
The Normans (since the Lombarsd didn't unite the place as one kingdom) count as "its own dynasty"? Please. They're as foreign as the HRE and more foreign than the ERE.
So the question is, what makes the HRE better off without Sicily? And how are the southern Italians better off as their own kingdom besides some whole a-state-for-every-people thing?
I'm not passionately for or against the idea, but I would like to see a reason why the best solution involves local rule that doesn't involve treating the HRE or ERE as a bad thing for whoever is under their rule.