Some of them are really lethargic, a few have swollen faces, there’s mucus galore, and a few have “spitty” eyes sometimes.
the ones that are sick are pretty sleepy and won’t leave the warmth of the coop.
one of my beautiful roosters sounds like a coffee pot brewing
One of the hens had a string of spit hanging from her mouth, another has died from this already.
I'm sorry you lost one.
That is quite a list of symptoms besides sneezing

Treat only the ones that show symptoms with the injectable Tylan50, hopefully it will make a difference. Try to keep the mucous cleared away and the goopy eyes cleaned up. For eyes, you can rinse with saline and apply an ophthalmic ointment like Terramycin or Vetericyn eye gel.
The one with the mucous coming from the beak, try to clear that as well. Check her crop to see that it's emptying overnight.
Encourage them all to drink, you may want to offer some electrolytes for a day, then switch to poultry vitamins for a couple of days.
I understand they want to stay in, that's fine. Sick chickens can also benefit from warmth, I see you have a heat lamp in the coop, but you will want to make sure there is plenty of ventilation/air flow so that fresh air is coming in the coop. Respiratory illness is spread through dust, dander, feathers, poop, mucous/bodily fluids, on shoes/clothes, etc., so you don't want to have "dead air" in the coop.
If you happen to lose another one, then consider sending the body to your state vet lab for testing - this will help identify what illness you are dealing with. http://csu-cvmbs.colostate.edu/vdl/Pages/default.aspx