It is true there would be many thermodynamic equations needed to actually design a working system, but I do know that if properly designed, the energy required to run the compressor would definitely be less than that required by the high pressure pump of an RO system. Also, the energy used by a compressor to compress the gas manifests itself as added heat in the gas. Normally this is considered a loss, but in this case it's the heat that you want, so it's actually helping you.
It is true that scaling is a problem, but I've never heard of an RO being used to pretreat evaporator feed water. Evaporators produce cleaner water than RO out of dirtier water than RO's can handle. The standard pretreatment is a dosing pump with a chemical anti-scaling and anti-foaming agent, though this may not be necessary for the size of unit we are talking about.
We're all here to be educated! An evaporator is roughly box shaped, whereas most boat RO's are long and narrow. The evaporator would not necessarily be 'bigger', just differently shaped and it need not be in the engine room. It can be anywhere you can run power to. The primary heat source would ideally be engine jacket water, the engine's saltwater cooling loop would not be interfered with (if you want to chance it, go ahead, but for all the reasons you listed, I wouldn't!). The evaporator is in a vacuum while operating, so it can draw in feed water with just a float regulator and not a feed pump, but it will need a brine pump to discharge the brine overboard. A nice addition is that the hot brine can also be used to preheat the feed water, saving more energy!