Yes it basically became pointless trying to come up with a rational explanation for these types of scenarios, and it would appear the system is still hopelessly broken.

Has UA been holding domestic PZ until after schedule changes are finished? I have PP down for some domestic flights for my traveling companion, most only have 1 person in J (me). Now that June might (finally?) be more settled, I'm wondering if PZ will start opening up. In previous months has this been the case?

I have not seen open domestic PZ until within a week, and only on lightly booked flights (of which there are not many).

Is it possible to use the plus points for somebody else on different ticket? Let's say +1 is flying alone and I want to upgrade her ...

Yes

Thank you! I assume I'd have to call for that because the app only allows me to use the points on my own bookings. Correct?

I don't know about the app, but if you look up their record while you're logged in to the website you can sponsor the upgrade.

No. Get her reservation / PNR (passenger name record) and enter it to apply your PP to her reservation.

David

Can I upgrade with plus points when operated UA but ticketed by LH?

that is tricky. Yes, united.com will let you do that. It can cause issues though on the backend when LH thinks it might want to re-issue the ticket or there are schedule changes or things of that sort.

Day of departure I don't think there are too many reports of problem w/ this, but there have been a couple of 'bad' failures in the past. Definitely YMMV and need to babysit the reservation.

I've done this successfully pre-PP with an RPU . The only issue I recall is that I couldn't check-in online. Had to call in. Took the agent a few minutes to figure out.

PlusPoints & flight change
 

Does anyone know what happens if a PP+ upgrade clears in advance, but you then need to switch to a different flight? Are the points refunded or are they lost?

Re-deposited.

With one minor corner case exception - for "Skip the Waitlist" the PlusPoints are forfeited on a change.

On my account page, under PP upgrades, it says "After meeting the requirements for Premier 1K, members earn 20 PlusPoints for every 2,000 PQP beyond 17,000 PQP."
On the PP page, it says "Additional 20 PlusPoints for every 2,000 PQP beyond 15,000 PQP"

Is there a typo?