The main problem with a single European army is one of authority: what would the chain of command look like? I think no single nation would cede control of their military to a supernational entity, so an EU army would look more like a UN task force than a cohesive army: multiple independent divisions operating under a central strategic command.

But yes, I agree (partly) with your assessment, there doesn't seem to be a lot of willingness in the Western nations to support Eastern Europe militarily (or sometimes, to even acknowledge they have a military). However, when it comes to it, they may not have a choice: an occupied Poland puts the enemy within striking distance of Berlin, so defending Poland is of strategic importance. The same applies (to a lesser extent) to Finland and the Baltic states, losing them means losing control of the Baltic Sea.

I'm not sure exactly how that would work out for Romania, from a purely West-European point of view I would draw that line across Slovakia/Hungary rather than further to the east, however as things as are now, Hungary and the former Yugoslavian nations would be a liability, so Romania may be essential to defending Italy and Austria.

That being said, I very much hope the above remains idle speculation.