Normally we'd tell you to go get fitted at a running store for shoes but at size 16 it's doubtful they have anything in stock so I'll recommend a popular shoe that comes in big sizes and works for me (size 14 myself).

New Balance Men's Fresh Foam 1080 V10 Running Shoe https://www.amazon.com/dp/B07RP21PW7/

Not only are they available in 16 but also in different widths. They also have a big toe box which is sounds like you might need given your blister and bruised toenails. The V10 are last year's model and run about $100. This year's is the V11 but other than different colors they are pretty much the same but about $150.

Make sure you keep your toenails trimmed.

For 3, the 9 inch boxerjock from under armor work well for me. Any extra long boxer brief that is part spandex should help. I also like having a longer pair of shorts with a heavier material.