A: Sort of. If you have more than enough cards in your hand to cover the number of plays left allowed by the Play Rule, then you just subtract how many plays you took from the number shown on the Play Rule. You played 1 and it’s Play 3, and you have 7 cards left in your hand? Play 3 minus the 1 play you took leaves you 2 remaining plays you could swap for draws. Your hand size does not affect how many plays you have left to swap.
If, however, the Play Rule indicates more plays left than you have cards left in your hand, then the number of plays you have left is the number of cards in your hand. The number of plays you can swap for draws is the number of ACTUAL card plays you could make, not the theoretical number of plays allowed by the Play Rule.
Draw 1, Play 3 is in effect.
You have a hand of 0 and you draw 1 card. Now you have 1 card in your hand. How many plays can ACTUALLY be taken by you? Not 3 because the Play Rule says 3, but 1, because you only have 1 card in your hand. You can’t play cards you don’t have. At whatever time you choose to exercise Swap Plays For Draws, the question is: how many ACTUAL plays do you have left? In this case, you have only 1 play available to you, which you could choose to swap for 1 draw. Now you have 2 cards in your hand, but do you get to play them because the Play Rule says 3? NO, because with Swap Plays For Draws, you are deciding to sacrifice ALL your remaining plays for draws, so, by definition, no matter how many you drew, you have no plays left in your turn.
This is turns out to be exactly how we figure out how many cards can be drawn when the Play All (or Play All But 1) is in effect. In that case, you look at the number of cards you have left (or that number minus 1) and that’s how many plays you have, so that’s how many cards you draw. Again, remember that using Swap Plays For Draws means you have no more plays left in your turn, so you won’t be able to use any of those cards you just drew until your next turn.
Swapping Plays For Draws is one of the ways you can avoid having to playing a card that would make someone else win.
See also: Can I use Get On With It if I played my only card, but the Play rule says to play more?