I suspect the reasons UA is unable/unwilling to seat you as you request have nothing to do with bassinet. Rather they probably are the usual cases: passengers in Y without status have extremely limited selection for pre-booking seats. When only a few seats have been assigned, UA cuts off the pre-selection process and leaves it until check-in. They will likely not waver from this policy even for a bassinet-requesting family.

I can share with you my experience with the so-called "floor bassinet" from 2 summers ago. When the FA brought it around to our seat after take-off, I thought she had made a mistake, accidentally bringing us the bassinet's storage bag instead of the bassinet itself. No, she assured us, this was the actual bassinet.

It was a vinyl/"leather" pouch with snaps up both sides. You were supposed to slide the baby in like an envelope. This then is supposed to lay at your feet on the floor, under the seat in front of you. It does not attach to anything (seat/wall/etc) in any way.

This thing looked both unsanitary and uncomfortable, but the FAs were helpful in bringing us extra pillows, blankets to make it more habitable for our 8-month old. By the way, had he been much older than 8 months, I don't think he would fit.

Be aware that in summer months, the plane will be FULL of families, many of whom will not have realized that bassinets can be requested. Be vigilant about confirming with everyone you see at UA that day (check-in, gate agent, flight attendant when boarding, flight attendants after takeoff) that you RESERVED a bassinet and can they please confirm it is on board for you. In the boarding and takeoff busy moments, FAs may give out bassinets to passengers who ask for them but have not reserved in advance, causing a shortage. Make a polite nuisance of yourself until they bring it to you.

As an aside, depending on the ages of your other kids, I recommend requesting child's meals for the UA flight. Our son enjoys them on international flights (more than I enjoy the adult meal anyway!).

OT: I also recommend requesting bassinet and child's meals on LH. They generally take good care of kids.

Have a good trip!

Mark