I'm sure you could get something like that to work. Depending how gung-ho you are you can also set up a ChannelObject listener on your desktop where you can pass it arbitrary code to execute then link to that in a sandboxed cloud notebook. Thus bypassing paying for cloud credits and still being able to use a nicer FE than 5.2 would give you. Obviously there are difficulties here: what happens when you kill your desktop kernel being the chief one among them. But assuming you can get a CLI kernel link via ssh (100% should be possible) you can revive that channel interface.

If you do develop a package like that (it'd be max 500 LOC to get a simple one up and running, I'd guess) please do post about it. I'm sure lots of people would want to hear about it.

I would expect the answer to be no, as the front-end underwent a big transformation in V6 (as I understand it) which enabled things like Dynamic interactivity. My uneducated guess is that it is this type of functionality that is crushing your 5.2 FE.