Eyecup - for me, so far........ it's been ok (had the camera 7 months) but there's quite a few out there saying it's fallen off and they've lost it.
Correct, there's no bigger replacement for the eye cup. I wouldn't say stray light is a big problem... but things vary in usage, if the sun's low in the sky, shinning on your face from the l/h side, then you'll probably see it intruding into the corner of your eye. That said I find the eye cup better at blocking stray light than the X-Pro1
Not sure RE min OVF length... I THINK I read 16mm, but I don't own that lens to check
Not sure how to answer the OVF blocking question... there's not a single lens (well maybe the 27) that you can't see at least a little bit of in the OVF, but that doesn't mean they block the frame lines, or even get in the way of visualising the scene - that said of course, if you're close focusing, then the frame lines shift to the side of the OVF where the lens is, but then again of course - if you're focusing that closely the EVF is probably the way to go anyway
I don't have the 16, but my HUNCH is that lens would be the most likely to interfere with a pure OVF view, it's a huge bit of glass and the frame lines would be quite close to the edges of the OVF, comparatively the 56 is also quite a large lens (for a Fuji prime), but the FL makes the frame lines quite small, so it doesn't interfere with them.
Ultimately the majority of times (as I'm sure you realise!), any stray light/lens in the OVF annoyances can be avoided by turning your head a bit, even if it's just to decide on exposure and focus, then you can return to the position you want for the shot.