what are the issues in collapsing spaced a/b omni to mono?
None if you use just one of the channels and toss the other. Possible mono compatibility if mixing them together, but why do that when you already have two mono recorings both of which are non-directional on their own? Mono playback might sound more distant than stereo, so if you know you will be doing this maybe put the mic(s) closer than you would have for a stereo spaced omni recording.
are there any negatives to using a jecklin disc for close instrument micing?
Limited isolation if that is important, may be visually imposing. Potential to sound good otherwise.
what are the negatives in using smaller discs than what is recommended?
Baffling not effective to as low a frequency as that achieved by a larger baffle, less spacing is possible between mics while retaining the effectiveness of the smaller baffle. Many here have made smaller baffles and mount the mics closer together / closer to the surface.