The point of these alliance relationships is to prevent an attack on these countries. The point is not to win once they're under attack.
Well no. A credible deterrent requires a credible capability. If a potential enemy thinks your forces are only for "wargames" or for "impressing foreign leaders" then, by definition, your forces aren't a credible threat and therefore aren't a credible deterrent. If the enemy believes your force cannot "win once under attack" then there is no credibility. Credibility is dependent on capability. There are a lot of Navies like that - they have platforms that mostly sit pierside, depend on foreign contractors for maintenance, and operate with poorly trained crews that don't practice war-fighting skills. The US Navy isn't one of those navies.

Now, maybe you can argue the force structure is wrong or whatever, but there probably isn't another Navy in the world that's underway practicing actual wartime tasks as much as the US Navy.

Besides; prepositioned material and airlift of troops are a quicker and cheaper method of reinforcing said allies than cruising with more than a dozen battlegroups on the seven seas with never more than one or two MEU in range for an as timely reserve (and they would likely wait till many more CVBGs are in the area before they'd actually dare to close in with Taiwan, for example.).
Naturally you preposition when you can, but that's not always possible and it's quite expensive to put all the stuff you'd need in every single country one is allied with - that's why the US uses prepositioned, preloaded ships with the equipment on board.

Secondly, you need some redundancy with CVBG's and other assets because part of the fleet is going to be in the yard and then you have the problem of geography necessitating an atlantic and a pacific fleet.

Again; the size and all is impressive, but the forces would look very different if they were really about waging major wars.
Ok, I'll bite - what would a Navy that was "really about waging major wars" actually look like?

Congress politics (including legalised bribery) and bureaucratic dynamics are the real drivers, not actual preparations for war.
Well, of course Congressional politics is a problem, but it is one of many and probably not as negatively determinative as you suggest.