I thought that you quoted the rule 2 (The minimum bet is the size of the big blind unless a player is going all-in) specifically to argue that player A (in my earlier example) should be allowed to raise because he is facing a fullsize wager because the current minimum bet was reduced to $90 after player C went all-in. If that was not your argument, then I think I'm not following you.

Do you at least agree that player A (in my example) is a player who (after player C has bet), is what we could say is "a player who has already acted and is not facing a fullsize wager"? That quoted rule then continues by saying that such a player "may not subsequently raise", meaning that he is not allowed to raise. The rest of the quote "an all-in bet that is less than the minimum bet or less than the full size of the last bet or raise" doesn't actually add anything meaningful because the fact that the player is not facing a full raise already means that an earlier player must have gone all-in with a non-full bet/raise (a wager less than the full size is only allowed for a player going all-in).

Did you read the Poker TDA Rule #47? It uses different wording but says the same thing: "an all-in wager totaling less than a full bet or raise will not reopen betting for players who have already acted and are not facing at least a full bet or raise when the action returns to them". Poker TDA uses the word "full bet" for the same thing as Robert's rules use the word "fullsize wager". Here' how that quote applies to my example: player C makes "an all-in wager totaling less than a full bet", and such all-in wager then "will not reopen betting for..." meaning that the raise action will not be available for "players who have already acted and are not facing at least a full bet or raise when the action returns to them", and because player A is a player who has already acted and is not facing at least a full bet when the action returns to him, he is such player for whom the betting should not be reopened.