Your question hinges on a very important point: Users should not presume any degree of subject expertise from the diamond moderators.
This is not to say that your moderators aren't experts in their own way, but it's rare for anyone to be an expert in everything, and being an expert in everything is entirely tangential to having the ability to moderate effectively anyway.
The Stack Exchange model is predicated on community moderation. The community decides what is good and what is bad, what it wants and what it doesn't want, and so forth. One user may not be an expert in everything (or anything, really), but with enough people the community can be.
Now to bring this back to the point of your actual question:
What is the criteria for Mods to decide, if the post to be deleted or undeleted or the next course of action?
For the most part, a moderator would (or at least should) be taking community involvement into consideration. If a question is posted and upvoted, but has received no answers or significant engagement, it's probably not worth the hassle of undeleting unless it's a clear case of self-destruction of useful content (which usually means the OP is deleting most if not all of his posts, which is a trend moderators tend not to take lightly). If you feel that such a question has value and also want it answered, there's really nothing stopping you from just reposting it (or something similar) yourself.
If, however, there already are answers, this makes the decision trickier: Since the answers have not had a chance to be properly voted on, moderators really have no criteria but their own guesswork as to what counts as a "good and effortful" answer that really should be preserved, and which ones really aren't worth saving. This may — and very likely will — result in posts slipping through the cracks due to moderators not being experts in all everything.
If you the community feel that there's significant value that is lost from this deletion, it's basically on you the community to
- cast your votes quickly and effectively to make it harder for good content to be lost (and easier for bad content to not distract from it)
- use your undelete votes as necessary to ensure that such posts get restored
Since this is still fundamentally a community responsibility, arguments for (and against) undeletion really should be made on meta so the community can actually judge its value (since regular voting is not possible) and what actions should be taken; flagging a moderator after there's clear demonstrable value to the community (in cases where there aren't enough privileged users who can act on it) makes it a lot easier for the moderator to make a decision when it does become necessary.