welcome to the community of smokemakers... by the way, your brother is right, he needs to get a more traditional rifle: one with a sidelock, maybe even in flint. there is no cure for the addiction- you can only treat the symptoms (usually with the purchase of just one more flintlock, and some extended range time). Nope: no twelve step program for us!!
as regards your question about the tarnish having a bad effect on the brass, the actual tarnish is perhaps a few thousands of an inch thick, so it's only bad if you think it's bad, and if you think it looks cool (i do as well) it won't hurt to leave it there.
as regards going from a 75 grain to 100, you may want to throttle back. Check out Dutch Schoultz' treatise on how to get the very best accuracy out of your rifle. his website is
www.blackpowderrifleaccuracy.com, and the booklet is $15, which sounds like a lot but is really money very spent: you will spend about ten times that in powder alone, and be no closer to a repeatable load, if you do it on your own (please don't ask how i know this :redface: )
you will be able to take deer with .50 cal with no problem, assuming decent shot placement. if you can't get a decent pattern, it doesn't really matter how hard you push the bullet. i suspect that a 100 grain charge would be considered a bit on the stiff side, but get the booklet, follow the instructions, and do what works best for you.
if you're not already, try as best you can to get real black powder. i prefer Goex brand, but that's just a personal preference and there are several good black powders out there. the substitutes are harder to ignite, not as stable, and cost more.
then, of course, you will have to take your rifle to the range as often as possible and practice, which will have the same effect as playing a counrty-western record backwards: your bank will return your reposessed pick up, your dog will come home, your brother in law will get out of jail, the bank will call and explain that the foreclosure notice was all a big mistake, and so on. Shooting black powder will also make you thinner, funnier, more interesting to be around, richer, more attractive to members of the opposite sex, and generally just plain more cool. (Shooting centerfire, however, just makes you deaf.)
good luck, and make good smoke