I decided to take the boring route for now, and bought another used ashp complete. I asked my old boss about diy multisplit, and he just shook his head.
He is one of the hvac guys for shure. He said that that I would be very lucky just to make 3 fans work as one, and even if I did I would never be able to match the effect that an original indoor unit would give.
I also asked him about making something else out of my 3 evaporators, but he wasn't very positive to that either. He said its no problem to make one machine run 3 evaporators, but again the effect would be nothing like an ashp. "Like a refrigerator using 1 kw doesn't give out 3 or 4 kw of heat in the rear. The computer part of a heat pump is what makes it effective, and they are sometimes very delicate. Cards can be broken just by trying things they are not made for, like other or broken fans."
He said if I absolutely wanted to make something of my own, I should take use of the seawater around me and make somthing water to water.
Jeff5may, I think I found something like what you are mentioning
here: 