Having a quick google it seems that I can indeed find some CX tyres to fit 27.5. WTB do a range so that's that one issue solved. Now I just need to see if a rigid carbon fork option would work for the MTB. I think it would probably make sense for me to use what I've got, especially as I'd likely lose a lot of money selling the MTB as it's not one of the main brands and it's virtually new and unused. I could sell my old Sirrus to pay for a fork so it'd all work out.
If that's not an option, then you guys have been excellent with giving me plenty of bikes to consider.
@Bonefish Blues -- thanks for the recommendation, I'll definitely take a look at the Stivvy if it turns out that fitting a new fork to the MTB is going to be too much of a hassle. I'm a little confused with my options right now, but it's nice to have the options to mull over!