So most of the answers to your questions are in here:
https://coh2db.com/stats/
https://www.coh2.org/guides/29892/the-company-of-heroes-2-veterancy-guide
(Note that the vet site is not up to date, but probably less than half of the units on there had their vet bonuses changed since then. I believe sections only had their scoped enfields removed with all of their other bonuses being untouched.)
To go over some of those answers quickly though:
-They'll win against both grens and volks at long range in cover to cover battles. They'll probably win against grens long range in open cover battles. Long range in open cover against volks is probably a toss up.
-Yes, they have better RA than grens (shown on the website).
-Scoped Lee enfields were removed as a vet bonus (again, iirc). They also had no actual affect on the performance of the sections (but they did take priority over brens...).
-3 sections with that kind of split of upgrades is fine. You can go with all healing if you want (if you don't care for the arty flares, that is), but don't get more than one flare squad.
-Nades usually aren't that map dependent I think (its not like they deny cover). They're usually build dependent. If the enemy is heavy on HMGs or you just want to catch them off guard then you should consider nades. I suppose crossroads really facilitates nade spam because of the super safe muni points and high resources though.
-Always extra man in my opinion. The fuel you spend on the upgrade will probably be made up for by the extra territory you'll hold due to the extra power the upgrade gives.
-Usually never. Mortar pit if its the right map (meaning you can protect it and it can fire on key areas), and if you really need the indirect fire. The bofors is never really built seriously. If you want to build it, go ahead though. The sim city style of play can be fun (for you, not your opponent).
-For all of the weapon + vet stats you're asking about, just look at the DPS graphs on the website. Note that the one for the enfield assumes the sections are in cover. If not, their DPS will be lower than what is shown on the graph.
