I don’t see any good reason it shouldn’t work besides the ABS failing. As far as temp control goes, I wouldn’t sweat it. If you actually cared about temp, the thing to do would be to heat the gas up as much as possible like in a normal rocket; the way the nozzle works is it essentially converts the thermal energy into kinetic energy (it’s not exactly that, but I say it’s permissible to think about it like this).
How much do you have already worked out besides the fuel / propellant? Rocket nozzle design is an iterative PDE iirc (method of characteristics is the name), so unless you want to literally design the bell shape, I’d look around for a software or something that does it for you.
I’ve got a compressible aerodynamics book that might come in handy. It’s been a while since I took the class, but I bet if I spent some hours reminding myself, I could probably work something out.
Also thermodynamics.
Briefly reading about your propane / carbonated salt water … can you explain to me how you intend to mix these continuously?
Regarding propane in the local atmosphere, I have a fun story involving the accidental discharge of probably 100 gallons of propane when a tank’s stem got stuck open and my father and I could not get the hose back on. It is still the loudest thing I’ve ever heard, so loud I had to cup my hands and scream at the top of my lungs directly into my father’s ear to be heard. It so happens that as a result I know breathing in tons of propane makes your lungs hurt somewhat.