I do not use stereo wideners at all, so I'm not sure what the issue is with the guitars. They were only panned to fill out the sound field. I suppose the tracks panned left and right are sufficiently similar to cause some phase cancellation. Maybe that's the issue.
Could be. Or perhaps you've used different processing on each side? A lot of processing choices can shift phase relationships as a side-effect.
I do have a question regarding your comment about the cymbals. Not sure why they would disappear in mono, when there is only one overhead track. How would you prevent that from happening with only one overhead track? Same with snare. All mono tracks panned straight down the middle. Not sure why switching to mono would affect that at all.
Fair question. In which case, that leads me to suspect the stereo effects you've applied to them. Mono-compatibility can be a particular problem with a lot of plug-in reverbs, for example. To check this, try soloing the effect return while switching between stereo and mono.
With the guitars all processing was exacto the same on both sides. I went back for a closer look, and they weren't time aligned perfectly. Fixing that fixed the phase problem.