About the fungible particles thing - an interesting tidbit is that physicists had hints about this way before QM was formally developed. If you look up the derivation of the sackur-tetrode equation, they had to reduce by a factor which removed the states which could be accounted for by swapping particles (e.g. because of their uniqueness via a label). This correction turned out to be absolutely correct in deriving the entropy of an ideal gas (and more importantly, validating the extensiveness of entropy). Gibbs new about this a long time before it was formally resolved - see Gibbs' Paradox. Fascinating if you ask me

Edit: and this was roughly late 1800s, for context

Power does not alternate only for 3 phase power (the math works out to have a literal constant power). In a single phase application p(t)=v(t)I(t) definitely can be an alternating signal, of varying complexity as the current changes its phase wrt voltage