Well you've got to understand that Rome during and after the Crisis of the 3rd Century rapidly decentralized; it was becoming more and more feudal and manorial, and serfdom was basically poking its slimy head out of the ground.
By the early 5th century, the Empire was shabbily glued together through systems of honor and--basically--vassalage. The reason this all didn't fall apart so quickly is the greater degree of decentralization, meaning less taxes and more local decision-making, which leads to a happier populace. Still, while this decentralization meant that civil wars were smaller and harder to embark upon (mostly ending in one decisive battle, whereas Caesar's Civil War and the Final Civil War + Era of Violence and Unrest lasted in the first case 5 years, and in the latter 14), it also resulted in great ineffectiveness when it came to maintaining native troops and protecting the borders.
Christianity and the Crisis of the 3rd Century's decentralization led directly to the army basically becoming a 100% mercenary and barbarian force (it's a lot easier to "hire" [read: buy off] a large group of armed men who have nowhere to go and are quite happy to make violence in your city than it is to send hundreds of recruiters hundreds of miles on crumbling roads and through bandit-strewn hills--and in the end, to an ambivalent populace). This force, while professional, had no problem with accepting a wealthy governor's gold and turning on their erstwhile Emperors.
So when the Germans attacked and the gold dried up, the myriad of feuding (and almost feudal--excuse the homophonic pun) leaders for these mercenary contingents, plus disloyalty in some of the more newly settle foederati, made this army a hilariously ineffective fighting force. Cue Benny Hill music.
So then you have a Western Emperor with his palace falling around his ears, and an Eastern Emperor whose taxes don't come from there, and therefore who cares quite little. What he's hoping for is that the West will get weak enough that he can grab rich Italy and North Africa (the rest of Western Rome was quite poor) for himself.
Aaaand that's sort of about it. Unless I missed something.
Which I probably did, this shit can get complicated.
So, what you're saying, is that the Western Empire was well along the path to what we could call the Middle Ages; with serfdom, vassalage, manorialism and so forth. (This is actually the assumption I made while crafting my Amalaingian TL, actually)
Would, perhaps the best way for a Western Empire to survive, be for it to 'fall' and be reconstituted in a new form; based on the old, but more adapted to current realities?
Other than that, I have ot say, I really am somewhat taken by the notion of, say, Romulus Augustus surviving to maturity, and being able to hold together a 'Western Empire" that really only constitutes Italy, and for this Empire of Italy surviving as a direct descendent of Rome, but never really able to expand its power outside of the peninsula.
But, maybe that's just me. I'm a bit suspicious of the notion that the problems of Roman Italy were easily fixed and that the Empire could rebound during this period if only a few things had gone 'right'.