let's say we have a cut that lasts exactly 1 full second. now consider the following scenarios:

scenario #1: the cut has 12 unique drawings in total, which are evenly spaced so that a new drawing pops up every 2 frames

scenario #2: the cut still has 12 unique drawings in total, however they're spaced in a "slow-in" way so that the unique drawings are tightly spaced in the last third of the cut (starts on 3s, ends on 1s)

what I'm wondering about here is: while we can clearly say the cut in the first scenario is animated on 2s, could we also say the same about the cut in the second scenario, as it has 12 unique drawings within a second? or would that be wrong considering the uneven spacing means each unique drawing doesn't last 2 frames -which is what the term "on 2s" implies- and we should say that it starts on 3s and ends on 1s, despite having only 12 unique drawing within a second

we should say that it stars on 3s and ends on 1s

This is correct. We dont use per second intervals, it would make discussing frame modulation very confusing.

Incidentally, all these scenarios would be related to timing, not spacing. Spacing, at least to how I feel is best, describes the drawings themselves. For instance you can animate on 2's and keep the drawings very close together, or you can animate on 2's with more exaggerated poses with large gaps in the arc of motion.