Kevin,
If you browse for just 5 minutes through the forum, you would notice that korm is the most helpful, thoughtful person you have ever seen on a forum! And the DT people are very helpful/competent, too. Let’s leave it at that.
Concerning the merging: I haven’t followed this thread carefully, and I can’t read the answers that korm seems to have edited out, but I wonder about the timing of your restore operation. In the past, I had system corruptions, too (not DT, but OS X wide). I immediately, i.e. within a day restored the most recent backup. It was still a little tricky to get back the one day worth of files, but that’s manageable.
Concerning your case: From your OP, I simply don’t understand what’s going on. You had a crash in December. So presumably you knew that in December? You didn’t act on it in December? So after the crash, you somehow moved on and added stuff until April, and then decided to do something about it? That sounds odd, so I assume I’m missing something.
In any case, if you now have the December DB running again, can’t you just open the April DB, and define a smartfolder that lists all files that were added or modified in the interim? Then you would copy those over into the December DB. Of course, if you had done extensive tagging/grouping on these files, you have to make sure that those assignments stay intact. That could be quite a bit of work, but honestly, 4-month lags between backup and restore is what is causing this.
Edit: I just noted more details in one of your followups: You EXPORTED items from the April DB. And then you simply overwrote the April DB with the December DB restore? Not keeping a copy of the actual April DB? You might still have it on TimeMachine, though. I would close the December DB, restore from TM the April DB to a DIFFERENT location with NEW name, and then open it. Also open the December DB and start copying files over. Of course, this assumes that this is a manageable number of files. I would have accumulated or modified probably a few hundred files during the December - April period. I would not want to use scripts to do any of that, let alone duplicate removal, as duplicate identification is a little dicey in DT. This simply sounds like a day of work, or a bunch of extra late night shifts to do this one by one. Of course, if you have dumped thousands of items during that period, I’m not sure what to propose.
The point is: If you use the April DB instead of the April Export, you can very easily identify all files that were added/modified in that period, and so your copy load will be much reduced and it will be much safer. The less files you have to deal with, the less errors will happen. PLUS: All DT metadata (e.g. URL from web clippings" will survive.