3. If swapping out the rear wheel would give 32-34km/h speed, that would be an easy and safe mod, even if the controller would still limit the power to a nominal 250W. But from what I've read, BH ask for the serial number of your bike and wont sell incompatible parts, or even matching parts without an explanation. This will put people off buying their bikes in the long run, especially if the competition sells their components more freely.
You are right if you live in Europe you cannot find a dealer that will sell you a US back Wheel but at least one user has found a US dealer that would sell him a back Wheel. I think I read about him on this forum. Try to search for the user here.
1. I wonder if the 48V Nitro battery would be damaged by the lower voltage range expected by a 36V controller, where it would not shut the system down until the 1 bar of a 36V system's low charge limit, which would be dangerously low for a 48V battery.
Dont know about this. You could ask on the australian forum.
2. Custom batteries or boosters scare me personally, due to increased risk of fire. I keep my bike inside my house.
I know speed is not much of an issue for the US market since most people feel 20mph is more than enough, but this is an issue for EU owners.
Recently all sorts of stores ranging from supermarkets to electronics stores have started to import cheap e-bikes that offer easy (in menu) speed hacking, have lax power limits and thumb throttles (no throttle on EU BH bikes, not even with appropriate retrofit parts, not even when local law would allow it).
BH owners like myself are seeing ever increasing amounts of social media posts from friends, posts in e-bike groups and forums showing off their fast and powerful, much cheaper kits and bikes