Missed some questions! Sorry about that.
I have not tried WIMboot. It sounds pretty fun to play with, but I know nothing about it. My gut throws up some warning signs though - my purpose of using a WIM is to have a compressed backup that is guaranteed pristine readily available. If you run your OS from that same backup, it seems hard to avoid the inevitable Windows degradation, which defeats the purpose for me. If they are saving changes to the OS in a different index, it might be OK, but certainly not if they are using the primary index of the WIM.
I don't use Bitlocker, so I can't say for sure what will happen if you do. My gut says it will be fine, but I have no basis for telling you it would be safe. Sorry.
Finally, yes, it will work just fine with SP3 (as it will with any Windows 8.1 machine). I would not inject the same drivers, obviously, so that step would certainly need to be adjusted, but other than that it will be fine. And I am glad you find it so easy to follow!