Oh wow. I'm ashamed to say that whenever I saw a database I just took a different route, and you just went and (almost) made it work. Thank you for your hacks. And, big respect! (and taking notes! ;))
What I can offer is that, after some minimal exploring of CopyQ, it looks more than promising. It's all about clipboard management, is open-source (available in brew too), useable from the CLI AND scriptable (in Qt Script/Javascript!). One of the example commands is "Paste and Forget", which sounds like it is very close to what we want; but it needs some adjustment, because its exact use case seems to be about first selecting some item(s) in its GUI palette. To be honest, the only problem I see in CopyQ is that it seems to include a wide range of kitchen sinks and I will have to go through them in case a LIFO was already there.
So, my plan before seeing your hacks was to keep using LaunchBar until its demo period expires in 2 days, and look into CopyQ in the meantime. Now I feel I should do it right now, but I'm too short on time :(. I will post again when I finally get into it.
For completeness: LB is supposed to keep working after its demo period, only with some nagging(?). So if LB's clipboard functionality keeps working after the free trial expires, then a possibility would be to just disable everything else in LB and stay with its "Paste and remove from history". (The things I don't like about LB is that every few seconds it uses about 9% CPU, and reads about 100 MB per hour from the HD, even with all of its indexing disabled; before I disabled it, it was still much worse). But now I'm pretty confident that CopyQ will be the long-term solution.