Good to know it (probably) wasnt me... come to think of it the similar may have happened on my wife's first FXDL many years ago.. so you're probably right.
Corroded parts... ugh. shopping fleabay now. Granted I dont "need" roller rockers... that doesn't mean I don't want them... LOL. I'f I'm pulling rocker box covers I'm not doing it for no reason other than swapping chrome.. (I'm just using it as an excuse!) I knew about the breathers from my service guy.. The "if youre going to open it up.. you can upgrade the.... " (Here's where the snowball begins... )
If the bike wasn't at the house in pieces I'd just let him take it and fix it. But he would have the same parts problem.
Exhaust: Thunderheader has a bearable sound. the bike currently has V&H slip ons which I don't care for. I'd rather put my spare set of fullsac CVO cored mufflers on it. I also don't want to wrestle with the duals with the crossovers. fullsac makes a kit to make it a true dual. My problem still comes with re-coating the pipes and heat shields locally. I'll look into the Fatcat or the Supermeg. The falsie isn't a "need, its a nice to have," the other option is trying my spare '09 pipe (why this is still hanging in my closet I don't know) and seeing how it fits (Or doesn't)
You're correct that the M&M is a love it or leave it system... it either runs flawlessly or is a headache for some.... Example: with basically stock bikes (My friend has 99k on his 2000 FLTRSEI) the only mod is Rhineharts, colder plugs and a "regulator mod" an old harley tech did for him to increase pressure. (I believe it involved compressing the regulator slighty to increase pressure, the same thing we used to do with turbo buicks BITD). He has changed fuel pumps a couple of times according to the service interval. Mine is still stock but doesnt have anywhere near the miles. Mine runs great as well.
I'd leave mine intact for the reasons noted. The T-max can be a handful because it inst mainstream (Enough) and the parts are not easy to get. much like running Big Stuff 3 for an automotive application.
The neutral switch ain't broken (yet) but being the equivalent of a bull in a china shop, its probably safer for me to have a spare available because I'm sure the dealer wont have it if I need it!
You're deep in the weeds now!!

I know that forest of weeds. Been there and bought the spare parts tickets

! We know, in our minds, that the enemy of perfectly good enough is sometimes the desire/need/2nd beer compulsion to make it just a little bit better. Sane people around us say, "you sure about that Bubba?" Then we merely act like we consider their ideas and warning before, BAM, it's off to eBay we go!!
Let's see, my old Road Glide..... Brembos all the way around, Delphi 04-05 so it had to have new instruments and cruise module along with everything else, radio.... Oh, God, I'm not starting this list. I'm just not in to that kind of masochism today. By the way, it's powder coated and diamond cut cylinders and heads that are part of its engine upgrade work (including the newer automatic compression releases) are all here in a pile. Some weekend soon!!

(gawd it's a sickness)
Ok, roller rockers. Expensive and ,unless you're really stepping up the cam, expensive to no real benefit. Plus you'll have to grind out part of the upper rocker boxes for clearance. Not a big deal. Just something else you'll have to do. Roller rocker money (if getting name brand pieces) could buy you have an exhaust system?
Exhaust choices are like religion. You pray to whomever brings you the most peace. If you try an 09 exhaust system that crossed under you won't have the mounting locations at the back of the oil pan necessary to make it work.
Hey, just thought of something else. While you're in the "upgrade" mindset here's a part to inspect carefully. Check out your swingarm. The pre-02 swingarms had a tendency to crack and/or twist. Make sure yours is ok. Look very carefully around where the axle goes through. It's worth an inspection at every oil change. You can change to the 02-08 swingarms to eliminate the problem but it will involve more parts than just the swingarm.