Just as a quick update: I feel a bit dumb, because when I actually looked at the objectives under my stereoscope, I think what I was taking for a delamination issue was instead unpolished glass surrounding the center of the lenses:
(I was also perplexed by what looked to be a frosted glass surface inside the objectives, which was just the projected image of the glass bottom of the brightfield/darkfield condenser in my gemological microscope--lots to learn here).
I am assuming that must be what this is, given the concentric grinding marks and such. No wonder there are issues around the edge of the image if you hold it up to your eye and tilt it!
On the plus side, I think I only saw a single spot of what looked to be delamination per the example posted in the 25x objective, and it was pretty marginal. Presumably the issue in performance on the 40x had more to do with hand holding a nikon head above a zeiss base than anything with the objective itself. They are all fairly dusty but no mold I could see either.
Thanks everyone! I really appreciate all the help and knowledge you've shared on this.