1) So far the benefits of the EU vastly outweigh the drawbacks (IMO). I think too much time and money has been invested in the EU by everyone involved to simply let it fade away. The newer members (mostly ex-Iron Curtain countries) have had to revamp their economies to adopt more stringently free market policies. They have had to open up their farming industries to competition, reduce subsidies etc - which has had an extremely detrimental effect on segments of the population, but will be worth it in the long run. In the countries which have been in the EU for a long time, the idea of the EU has become quite accepted (and, for one, I personally like the fact that I can live and work in any EU country without needing a visa....)
2) Where to start! France & Germany would like a highly federated EU - with common laws, taxation,defence & foreign policy etc - in effect a superstate, similar to the US. A state like this could (in theory) be as powerful as the US on the world stage - hence why they want it. However, because this could potentially make the EU a rival to the US, Britain, Spain and Poland (amongst others) are strongly against it and want a far looser union, where defence, taxation and foreign policy are decided by individual nations. Countries near Russia worry about how the EU will deal with Russia - they would like the EU to keep Russia at arms length. Countries geographically far away from Russia would prefer to adopt good relations....some countries (Ireland, Sweden and Austria) are commited to neutrality - so want to be able to decide their own foreign policy etc....
Most countries have SOMETHING they will almost absolutely not back down on - so the EU has normally accepted a compromise - excluding that country from the relevant requirement. Right now there aren't any disagreements that will split the union - but there may well be in the future, especially over how 'strong' to make the union.
3) There are two views.....if you think that the EU could potentially become a threat to the US ( a bit of a stretch of the imagination right now - China is a more likely competitor to both) then anything that weakens it is good for the US.
On the other hand, it is in the interests of the US that Europe remains at peace, so that there isn't a repeat of WW2. This peace has so far been guaranteed by US troops in Europe. If the EU were to become a stronger, closer union, it could probably get together enough troops to defend itself (at the moment, only the UK and France have got any sort of serious army). Then the US troops in Europe could go home - (or go straight to the Middle East maybe!).
However, the US is committed to remaining the military super-power of the world and preventing other countries from challenging it, so any EU force would have to be too small to actually challenge US forces - whereas
certain countries in the EU would like the EU to have an army big enough to challenge the US - not to fight a war, but to prevent perceived US interference in other countries.
The EU is one of those complex issues that takes up too much time and effort!