Swapping crank and cassette

Your cranks are probably compatible and you can just swicth it over. But without knowing for sure which BB you have there is no definite but IMO it is an easy jobe and would take more than 10min to figure out by just pulling off 1 crank arm on both bikes.

I would not mix and match cassettes although for just the 11 to 12 you might be able to do it.

My suggestion would be to just swicth chain, cassette, and cranks if you want to go through with this. Mixing parts that have not been running together is asking for some issues. This way you are not worried abotu cahin length, wear from the system or anything

1. You should be able to just swap out the cranks.

2. You can, but how well it works depends on differences in wear. If one is significantly more than the other, the chain may jump some. Also, switch the 11 & 12 over. The 12 on the 12-25 will have bits for the lock-ring to grab onto, while the 12 from the 11-28 likely will not. I don't know how much difference that makes, but better to play it safe.

3. You're going smaller, so there is no chance of there not being enough chain. It's possible there may be a touch extra, but it depends on how well it was sized in the first place. Too much could flop around some, but see how it goes.

from my own experience doing the exact thing you are considering, brigby1's advice is spot-on. If the race is hilly enough to worry about it, you will have some good descents as well. If you are used to a standard/12 set-up you will definitely want the 11 for big descents. While you won't technically spin out with the 12, it will feel like you are compared to what you are used to. This is the exact same set up I run; it gives you a bigger gear but allows the ransitions in the middle of the cassette to stay close.

Thanks for the help everyone. The crank swapped over no problem, just had to adjust the front derailleur. I can't quite get rid of the rub when on the small front/big rear but I can live with it.

I'm going to stick with the 12-25 for the race. I never lead on the descent in a race, that's for the bigger guys. I tuck in behind for the free ride. If I lead, it's not going very fast because I'm not big enough.

You did bring up about the similarity of the cassettes. Even if the one was 11-28, it's only the bottom 3 gears or so that are different. The 11-28 has a bit bigger spacing on them, but they both start skipping with the 16. That drives me nuts, as I'd much rather have that gear and have the 3 tooth jump on the last couple.