First, anything you send needs to be much, much, much shorter. I would frame this as two separate claims.

1. Bags did not arrive as scheduled. All that matters is that you checked them in and they did not arrive. All of the back-and-forth over 2 days is irrelevant. You are covered for your interim purchases up to approx. $1,660 per passenger (as noted above, UA by policy will not require receipts up to $1,500). Thus, simply submit a list of your purchases which would include replacing the presents, and you should be well taken care of unless there were items of extraordinary value.

2. The downgrade entitles you to a refund of the fare difference. UA can calculate the difference when provided with the PNR(s) and the four e-ticket numbers. The location of the seats is not relevant nor should you refer to it as Basic Economy as that is not the case and will only slow down the process. As you later note, you have already received customer service gestures totalling $900 per passenger.

3. I would forget about the meals. It simply is not worth it and detracts from the rest. For the benefit of others, if you have special meals on order, always check with a FA at the time of boarding. If, for some reason, your meal did not make it ,there should be time to remedy the situation. Once in the air, all you can do is make do.

For four of you this will work out to $9,600 in compensation + gestures, $6,000 of which is cash. That is not a bad result and could come close to a nice trip somewhere for you.