I have the f/2.8 55mm Micro Nikkor, when I have the PK-13 extension ring on, the edge of the lens is quite close to the subject when focused, I doubt you could use another extension ring to get greater magnification without almost touching the subject.
For some time I photographed postage stamps for a friend, did this on and off for about 5 years. I have three automatic nikon extension tubes, they have their uses, but are not as versatile as a set of bellows; but that is another story.
I've used my F3 without a prisim many, many times, I cannot ever remember light leaking throught the focusing scren to be an issue. Possibly because I almost always use manual metering, sort of cannot remember the last time I used automatic metering. This could be a function of the light meter taking its measurement through the pellicle section of the mirror. If you take the lens off, inspect the mirror at different angles, you'll see a darkened circular section in the centre, behind this is the light meter. If you slowly move the mirror lock, you can see the light meter behind the mirror.
I don't know how other cameras from other manufacturers do their stuff, but the F3 is a wizz at getting very accurate reflected metering.
Mick.