Darby
I don't know that I can really tell you whether or not you should reopen your other town, but I can answer a few of your questions.
First, you don't need special stumps to get mushrooms in November. During November, 5 mushrooms will randomly spawn around any trees and one of them will be a piece of the mushroom furniture set when you pick it up. Special stumps are only needed if you want to spawn mushrooms outside of November or if you just want them for decoration.
Second, if both of your towns are at 10 villagers, then you can safely travel between them without picking up any voided villagers. If you go below 10 villagers in either town, then there is a chance you could pick up a voided villager in the town that is at 9 or fewer villagers.
It's really up to you if you feel like going back to your old town for any reason. Juggling 2 towns is somewhat time-consuming but having access to the items in that town could give your new town a boost. I'm currently managing two towns, but my main town is basically complete and I just log in to do daily maintenance.
SCORPA15
It's hard to tell because the house is showing up slanted, but you might have room. My suggestion is to ask Isabelle if you can put a 3x3 PWP in that spot. The fountain is my preferred option. You don't have to go through with building it. Just ask her about the location and then tell her you've changed your mind and cancel before she sets it up. If you can build a 3x3 PWP there, then you can likely plot a villager in the spot. The only different that you'd need to account for is the doorway which requires an extra space in front of a villager house.