She's right and you can only pre-assign seats if you are in J or F.
That's not quite true. I have an upcoming LH flight next month, booked through UA, Q fare, and I can change my seats on lufthansa.com just fine using the LH record locator.
From what I understand, issues occur when the fare class you're booked into won't allow you free-of-charge ASR. I've had trouble last year with S fare tickets not letting me do ASR. Then again, their IT systems seem to have gotten an overhaul, because I also had issues getting a seat with a J fare last year, and that included not being able to get one during OLCI or even at the gate.
I will obviously check in with UA at the T-24 hour mark. Not I was wondering what will happens to the seats on the LH legs. Can I login to LH's website after check in with UA to select the seats that I really want or will I need to call LH to try to get a bulkhead seat?
Assuming that you've tried ASR and failed on LH.com, you can try the LH website again at T-23. Failing that, your next step would be to call them. Make sure that when you log into LH's site, you use the LH record locator, not the UA one. You can find it by logging into UA.com, viewing your itinerary and clicking on the "show additional confirmation numbers" link at the top.