It's likely that Ducky's family aren't the only swimmers in the Valley so it's completely possible that Ducky's mom has found a new mate. As for his absence, I agree that Universal just didn't want to hire another VA, even if this state of affairs is a bit awkward, especially as the scene didn't contribute to the plot.
I have to disagree about the amount of adult speaking roles. After all, large parts of LBT 11 and 12 was about Topsy/Tria romance and Grandpa even had a song (If Only) in the former. As for JOTB, the adults actually spoke quite a lot.