1. I attach recorded comment to one slide by clicking/selecting the slide and then start recording. The slide view time expands to match the recording.
2. I find that I can run a recorded comment over several slides by selecting the "Add sound to entire slideshow" option. The drawback is that if any slides are moved or deleted that impacts the position of the recording whereas if the recording is attached to a single slide, the recording stays with the slide if it is moved or deleted. My only solution is to set up the entire slide show, be certain all slides are in desired position and do the recording last.
. . . . My only solution is to set up the entire slide show, be certain all slides are in desired position and do the recording last. . . . .
You should find that, although the slide the recording is adjusted to fit the recording length, it is not actually attached to the file, in Timeline mode you can move the audio and change the length of the slide that was lengthened.
However it is not possible to select multiple slides and have them adjust proportionally for the length of the recording.
Your option 2 is the way commentary is often done, timing your commentary to match the slide changes.
An alternative would be to record the commentary first then adjust the slide lengths to match where necessary