Unless there is a recent Consumer Reports article on dress pants, any stories about the durability of particular dress pants likely will be limited in value. Therefore, choosing heavier wool (not cashmere) in a fit that isn't too long or too tight anywhere and laundering carefully are the simple guidelines for getting years of life from dress pants.
If it's neither male-pattern baldness nor alopecia areata (which usually looks different), chances of regrowth are excellent, with the right changes. The dermatologist's job is to figure that out (cause and probably active treatments). Minoxidi/Rogaine all over the head might be the wrong approach, one reason being that so much of it could get into the bloodstream and for example lower blood pressure too much.
No hat might help if there's a scalp condition. The wash-less advice lives on the Internet, but there's almost no validity to it. Only an extremely irritated scalp might benefit from shampooing less (and it probably would be smarter to switch shampoos and rinse better). For most men, washing less increases the risk of hair loss.
No, to the second question. For informal use, one of the few rules is to not mix solid black with solid brown. With that suit, various interesting possibilities exist, including wearing a navy cotton belt plus a brown shoe. Black is too formal, agreed.
(The website might be down now. Otherwise, there's an article about belts worth reading.)