Is it possible to re-enter the Pillinko area in Hollis's mind when revisiting it with the brain tumbler? I'm missing some figments and I remember there being figments in there, but it seems like there's no way to go back in.

I don't believe there is at the moment. I believe there is a post on reddit about this same issue. It has been reported to double fine.

I don't think you need to. I missed one and in coming back to the Pharmacy, I found a new one right in the central area. Painfully obvious in placement.

My hypothesis is that the missed ones get ejected from the Pillinko game after you win. Can't repeat it to confirm, of course.

From my understanding this is exactly how it is supposed to work. Figments in places inaccessible to the brain tumbler should be shunted to places you can reach. I'm pretty certain I missed at least one in the Pillinko machine myself and it was placed extremely obviously on returning via the tumbler.