I'm nervous about the air cells and "shrink wrapping". They seem big enough, but I want to make sure they aren't TOO big. Can I add a little humidity now, just to make sure?
Sure increase the humidity now if yo want, it does help them hatch easier... But don't expect the air cell to shrink, it's far too late to compensate for that at day 18+, you need to make those adjustments the 2nd week...
Should I do anything with the vent hole plugs? I read somewhere that they need to be open during lockdown/hatch. What do you think?
Yes, you should have some holes open during hatch to allow fresh air in for the hatching chicks...
How about temp? I assume I just leave it consistent as it was, around 100-101 thru incubation. Does it require any adusting?
Leave it alone, you can cause more damage by spiking it low or high trying to adjust it, with no benefit... Some people lower it a few degrees or move the eggs to a hatcher that is slightly lower temp but it's not necessary to do so at all...
I assume it might need a little lowering as the chicks heat up in there.
Nope they will be perfectly fine at that temp, when they first hatch their body is not able to properly regulate heat or produce heat reliably, thus they idealy need that 90-100°ish environment initially, until their normal body functions start to kick in in the coming days, their normal body temp at that age is 103°-104°...
Hogwash, Hyperbole, and Paranoia... Open it if you choose whenever, and don't stress over it... If the humidity drops a lot get a spray bottle and fill it with warm water, and every time you open it spray a few mistings in the incubator before you close it back up...
I might be able to leave them for 2-3 days without losing my mind, but is it ok to check on them with some candling if they haven't started hatching on day 21?
Candle or check them any day you like day 1 to day whatever, candling causes no harm as long as you do it with clean hands and take care and don't keep them out of the incubator for hours at a time...
I candle regularly on day 20 and 21 to see when they internally pip...
After the first ones start to zip, I love to take the others out and hold them to my ear and tap a finger nail on the air cell, if they have internally pipped you almost always get a chirp response back...
Also, links to any threads that can help me figure out why these 5 quit on me would be great.(I'm going to assume they were all roosters so I feel better about it.) I suspect bacteria. Some of them were dirty, and some had shiny spots on them like a greasy finger print in the bloom. I wonder if that wasn't a farmer with dirty hands... (I purchased these from the feed store)
There are some threads and sites that cover eggtopsys in detail, Google away but remember that some just were never meant to hatch...
I feel with you. I'm hatching a mere dozen eggs and for some reason they are all quitting in me! I'm down to 7 tonight that are still alive an dkicking. Humidity is good, air cells look good, my temp was a little low at 100 but constant and I've been turning them at LEAST 3 times a day. No idea what's causing them to die (Three quit at 10/11 days, and one I think must've quit yesterday or today (day 15/16). 
I drowned a hatch last year, then I went with the dry method and got the best hatch I'd ever had. Are you using the acurite "humidity monitor"? I've used that one and it's great. 
