You need to used Research Grade Propane
co2 is simply more easily purged, but it is a solvent in chemical terms.
all the time..it's highly polar though, so it's necessary to do a quick wash or QWET. It depends on the product, but it usually ends up slightly viscous. Although, I made some with white strawberry cough once that ended up like dry pine sap..very nice and it burned very clean. This was done without winterizing and decarbing..amazing terp profiles.
an alcohol still is what you would need, as ethanol needs a higher temp to boil than butane. heating the alcohol might include some decarboxylation if your not careful with your temps though. not a bad proposition if you want to make a topical or sublingual though. it's quite a process with ethanol, takes several washes, but through winterizing and using a vac assisted #1 filter, you can achieve a very pristine oil, for med purposes while retaining a large amount of terps. If your more on a commercial scale, I would think butane and purging might be more effecient and less laborsome.