But how about not scheduling swapped out processes if there currently is no free ram for their current RSS? of course kernel cannot know that a new process will balloon to eat all RAM, but once it has done so, is there a way to let it run to completion without being swapped out to "improve responsivity"?

There is no actual swapping in the modern kernels. Nowadays, it is paging, when the kernel pages out individual unused memory pages, not entire processes, so it keeps all non-blocked processes running, but only necessary memory pages in the memory.