The night can be as short as 8.5/9hrs and that might prevent moving her WU later even with a 9pm BT. It might be ok for 1-2 days tops to leave her nap uncapped but after it's either NWU or EWU again
I think you've answered your own question here
Don't forget, you are the mummy, you know her, you are the one who has been dealing with this throughout and you are the one who knows what is likely to happen. Trust your mummy instinct
If you believe it will only work for a couple of days then do that. All the rest of us can do is look at times and make guesses based on our own experiences and threads we've supported in the past.
hen if she naps for 1.5hrs she will WU at 3/3:30pm and after a such a long nap she will probably need 5.5-6hrs of A which means brings her BT to 8:30/9pm. Do you think I should just shoot for that right away?
Yes why not? Just take advantage of the opportunity given to you in that later WU.
when we travelled to a different time zone but somehow her WU keeps moving earlier and earlier and that's why I keep moving her BT earlier and therefore her whole day shifts earlier and we are back to 5/5:30 WU.
Well, here you have also said it yourself, you "keep moving her BT earlier". If she can only do a 10hr or 10.5hr night then that's all she can do. It's not likely to change until she drops her nap completely so you are stuck on a perpetual cycle of her waking "early"(but chances are this is not "early" she has just finished sleeping and cannot sleep any more) so you put her to bed earlier so she wakes earlier so you put her to bed earlier. If you work with the times she *can* sleep BT can be later, it can.
If you want 7am WU then consistent 9pm BT (or even past 9pm initially to force the body clock to shift) should give it. If you feel 9pm is just too late for you then aim for 8.30 (that is after her body clock shifts) but don't expect WU to be later than 6.30am. *If* you get more than 11hr night then great, if not, well I'd just accept it. I know it sucks but you can't make her sleep any longer than she can sleep.
I do think getting past 5/5.30am is the hardest part, and you *might* end up with a slightly longer night as a result but I'd keep expectations low.
In fact, I know a lot of BW mummies say to do a 10 hr day and 14 hr night after nap drop...I'd actually wait until she showed she could do that before doing it. I'd personally look for more like 11.5hr night (making more than 12hr day!!) initially and then FX the night will lengthen to 12 over time. If there are a few 14hr nights then fine but IME with DS it wasn't more than a couple of random nights like that. I was happy to get 11.5 and even happier when it turned out to be 12hr.