1. If there is a concert in US, you won't be able to get into that park. I don't think the exact dates have been released yet:
https://www.universalorlando.com/web/en/us/things-to-do/events/mardi-gras
Last year they had concerts most every Saturday and maybe every other Sunday as well:
https://www.themeparkinsider.com/flume/201901/6533/
2. If you make the hotel reservation directly with Loews/Universal (rather than a 3rd party or package deal), you should be able to cancel it with no penalties usually up to 5 days or so before your stay. So there is no reason to wait on making a reservation. But to answer your question, in general, the rates don't usually go down as you get closer to the date, they go up. Yes, APH rates may or may not have been released yet for your dates (and you may or may not ever see them for you dates), but if you've made a direct hotel reservation and you find a cheaper rate, grab it and cancel your original reservation.
If you go through a 3rd party or purchase a "package deal", you need be very careful of the cancellation/change policies as you could be locked in with penalties if you make a rate (or any other) change later.
3. In general, you aren't going to see major deals on AP purchases. You might find something during a few special "purchase days" like Black Friday and such, but other than that, I don't remember seeing much in AP discounts. And do note, it is possible the AP prices could go up if you wait around too long.
4. Depending on your spending patterns, there can be benefits from having one person with a higher level AP (i.e. food discounts, etc.) So you might look at the benefits of the more expensive APs and see if you purchase 3 Seasonal Passes and 1 higher pass, you might save more than the extra cost.
5. One more thing, since all of you will have APs, in years past, Universal has offered AP holders the opportunity to sign-up for participating in one of their afternoon Mardi Gras parades. We did that a few years ago and you ride the float and throw out the beads to everyone as the floats pass by. My daughter loved doing that. It gives you a much different experience from just watching the floats go by.