Coming back (way late) on this- it seems like the big benefit of the Ranger's best mastery is providing his prof bonus to the animal. Them having to sacrifice an action to control the animal is, as far as I can tell, the default for any humanoid-animal relationship. I can't find a specific rule that says otherwise, but I would be really happy if I did see one.

It very well may damage weapons. Steel has 10 HP so if it also takes 5 hp per round then that is definitely only 2 rounds. That makes it an expensive move, but not an impossible one.

related, not a duplicate. That question SPECIFICALLY focuses on torches and whether they can be used as a blunt object in addition to doing fire damage. The answer is kind of a punt because torches ultimately aren't made to be used as clubs but it doesn't answer the question quite how it's put here.

To clarify- nobody is assuming the PLAYER controls the dog. The dog ordered to stand next to the knight and protect him can have some specific combat benefits (Protection fighting style, possibly sneak attack, hex could help the dog knock foes prone), but it can also use its mere presence to dissuade foes from moving through a specific space to avoid opportunity attacks from the mastiff. So this is more about strategy than number stacking.

Crawford recently told someone else w/the same question that Wuxia weapons in the DMG are allowed to any character and then hedged by adding "with DM permission," which really answered nothing.

only thing I disagree with is it's 1 player "gaming the system" and 1 player who found a gap (which I don't consider the same), it's not our whole party (heck- I was in the barrel lol)

This answers the question in a way that's too literally- you mean the D&D monster "pixie" while the person asking means the general fantastical fairy "pixie"-- Quasit or Imp familiar can fit that role very nicely with fey patron warlocks (minimal refluff required)

Recent Twitter answer from someone who asked this same question said the Wuxia weapon names in the DMG are available to any character. No further information was provided.