Can I use my road shifters with mtb derailleurs? Or the other way around?
Yes, you can! This is done quite a lot by Di2 users who want to run a 1x setup only (no front derailleur) – on their gravel bikes for example. Have a look at the compatibility charts – they list which derailleur models are compatible with which shifters.
Generally speaking, the only real limitation in a modern Di2 system is that all derailleurs on the bike must be from the same category. This means that all derailleurs must be mountain bike derailleurs, or all derailleurs must be road (/ GRX) derailleurs.