Wow. That's a lot of thought and consideration just to go from 172.5mm crankarms to 175.
Just get whatever gearing you want, and if it's different from what you already have, make sure to get 175mm crankarms if you have to change the cranks. If you like your current gearing (it's not clear if you do or not, just that you don't like the 172.5mm crankarms), stick with what you have. Or if you can, just switch out chainrings on the current cranks.
If you change to a different model crankset, yes, you'll likely need a different bottom bracket. And yes, you can put a 34t ring on a 74mm BCD crank-