Love the questions!
1. Because changing the sides independently created problems that, to solve, fell out of scope for the Jam. It creates problems where you have to define where the "center" of a tower is, meaning that you could effectively move the towers by scaling different sides at different times. This is bad because:...
2. Being able to sell or move towers completely trivialises the scaling mechanic. I do think that if we had more time we would've made "how you're meant to place towers" more obvious/accessible to the player, like letting them preview how the tower scales before confirming placement. We experimented with selling towers but it turned the game more into just "Another Tower Defense" rather than leaning into the scaling theme of the Jam. Part of the interest/appeal of the mechanic for us was the puzzle-piece element of trying to fit your towers into awkward shapes.
3. They aren't technically useless but the game as a whole did not have much time for balance or conveying this mechanic as well as we would've liked. The size of enemies directly correlates to their health value, and the Bigcraft you earn (the scaling resource for towers) is directly related to the enemy health at time of death. This would force the player to try and use the big pools to earn more resources against easier waves so that they could successfully scale their towers to do enough damage in the late waves. On the flip side, if the player DIDN'T scale their strength enough, the small pools would work like a panic button where they could weaken enemies (at the cost of earning less scaling resource) if they were being overwhelmed by a wave.
We only had the finished playable product in the last few hours and tried to make sure the game wasn't too hard. Luckily the pools were incredibly simple features to include and so it's not like we lost time including them over balance. We just ran out of time.
thanks for the feedback!