He's a 1 year old vet. He can't be a free agent just after being cut so he was due to waivers where Carolina sits really low on the list. They liked him last year and probably think a 7th/conditional 7th is a small price to pay for a guy they were high on just 16 months ago.
Not Fox. What you just said is something EVERY team does EVERY week during the regular season. Each OC starts a game with a list of 10-15 plays they decided during the week based on film study & opponent and they follow through unless there's a major turn of events (QB injury, going 14-0 down after 2 drives, etc...). They don't change the approach because they go 3 & out on their 1st drive but after those 10-15 plays they've gotten a feel for the game (well, at least the good ones) and can live call the plays/adjust based on what they've done/seen so far from the booth/sideline.
It Williams at LT fails it won't be because of his weight. To quote G-Man: "But when you look at players, one of the things you have to say to yourself is, if a guy doesn’t have all the physical qualities, what is he missing? Let’s say he doesn’t have the pretty left tackle feet. What’s going to allow him to overcome? Is he especially powerful? Is he meaner than a rattlesnake? Is he really instinctive? There are plenty of guys who are playing in our league at a pretty high level who aren’t classic left tackles and don’t have all the classic physical qualities. But they have something that allows them to overcome…Can a left tackle come in and play? Sure they can. Just like quarterbacks or receivers. But it takes a mentally tough kid to come in and play now. And obviously having the skills.”