Ok, upon further investigation...
Flight originally booked as two people (1K and no status) on single PNR, departing tomorrow. Checked in this morning, did NOT split the record as we were both already in F the whole way.
Called in to get my name fixed. Res agent fixed name on ticket. Get email with new boarding pass, for just me and not companion, while email earlier in the day had both of us.
Check reservations on UA.com, and now I'm on a NEW record with the correct name, and companion is on original record and no longer on the upgrade list for the return flights.
Went to see if I made TSA pre using my previous method - opened up the United app, under the boarding passes found my itinerary - it still has a 2 in a grey circle next to it, indicating passes for two people. This is the only item on the list matching my itinerary. Open up the boarding passes, we're both there, and my boarding pass in the mobile app still shows the OLD record locator and old name and, when barcode decoded, indicates I don't make TSA pre.
Then notice that in the upcoming reservations section, the NEW record locator is listed. So I click that, then click reprint boarding passes, then click reprint boarding passes again, then select mobile boarding pass, screen save that, scan that image and...
Name correct and now DO get the 3 instead of 0, so the boarding pass will get me through TSA Pre tomorrow.
I do now also see the NEW boarding pass in the mobile boarding pass list of the mobile app (old one still there too) so I'm guessing the app downloads the passes and then doesn't clear them out if they change. (Sort of makes sense, as then you still have it there if you lose connection to the internet or what not.)
So at least I've proven that if the name on my ticket DOES actually match, I will get TSA Pre to work.
Now to get my PNRs put back together for the return, and figure out why, despite all records apparently having my full middle name, my tickets still come out with only my middle initial.
BTW, is United IT hiring? I couldn't do any worse than this.