Fecal testing in your case I don't think is absolutely needed, but it's better to do that for both frogs to be sure and have a piece of mind. Worms are not a reason for your problem. Usually that test is done when you just get frogs.

waxworms are great for your sick one, they are fattening and just what the guy need. When you take him to a vet, im sure she'll assess the situation. Ask her about if you need to force feed him, its easier to see that in person how bad the shape is and if he needs to be stuffed asap or it still can wait.

Usually sick frogs don't eat, just like humans, when you're sick you dont really hungry, the problem with that is without nutrition the immune system and all systems really are starting to shut down, the body just don't have enough strength to fight what it needs to and we have to resort to force feeding, your vet should assess if a frog is there or not yet. Ask about it.

Moss - yes in theory it should increase humidity, but do anything for a bigger tank you need to have tons of it and keep it moist at all the time ( not happening ). Moss if ingested is one of the main impaction reasons. Even if you feed in a separate cage, the frog might ingest it while shedding. So really potential benefits are minor when you risk dealing with impaction in frogs. Better be safe then sorry.

Yes, try foil and cover more, if you still not getting it, then perhaps you should either invest in a fogger or mist the tank more.