In this case it was a Patch, and obviously I would not get to draw an extra card as a reward for patching the timeline, and the card I’d attempted to play (whether a Patch or an Inverter) would be discarded, not put back in my hand, but we couldn’t agree on whether I could draw a card to replace this second play given by the Really Fast Time Machine. Since the attempt to play a card had been canceled, would the resulting draw to replace also be canceled? We didn’t think a Memo should decrease the size of the targeted player’s hand, but we weren’t sure.
A: Yes, you would still get to draw to replace your Memo’d Patch/Inverter. Usually, on your turn, you draw and then play, to keep your hand size unchanged. Normally, if someone had memo’d a Patch/Inverter played as part of a normal turn, the target player would have already drawn the card that keeps their hand the same size. It’s only because this is a special extra action that you end up drawing after you play, and it’s done to maintain the correct hand size.
With that in mind, it’s the playing of the Patch/Inverter which has been Memo’d, and not the corresponding draw, which is more a function of the Time Machine. As you point out, memos should not decrease the hand size of the target.