OK.. where to start... So in a fix application with limited charge ( like a fridge) yup good to go.. yo get everything rated no problems there... isobuten for that application go for it..

Propane.. so if you ues in splits and heat pumps is a problem. ( yes they use them in gas plants but everything is explosion proof ). As you can't have limited charge depending on tonnage. And Propane has a very very nasty habit of settling to the lowest spot ( heavier than air) so if you develop a leak in your system your turning your house in to a bomb.

I do think CO2 ( some adjustments would need to be made and a buch more work for welders ) will work good on the commercial side. ( grocery store racks and the like. ) I am not sold on it residential. --- would work better where most buildings already have cooling/heating loops with glycol. Witch in not north America.

And on a note for NG ( methane) the pressure is less than .25 a psi in a home and it is lighter than are so tends to float up and out if leaking.. and it smells.. ( do to an oderent. ) On that note the Propane that would be used in these types of system would not have that smell so even more of a worry.

Hey there your friendly neighborhood ref tech. R600a and R290 are straight up bad ideas ( more than a wee bit flammable (isobutan / Propane) ) and although r744 is good ( CO2 ) it's pressure are nuts. ( the whole system needs to be made of steel/iron) and would not work well with most common split systems. So ya nice idea I am not sure what's next but these are not the answers in private residents.