It could. Well aligned teeth tend to support each other. So if your lower molars and bicuspids fit tightly together when you close (with the aligner out), keeping the uppers in place may help keep the lowers in place as well. However, better to have a retainer in the lower.
Possibly. The lower teeth will function on the plastic. That's not a good fit. However , if worn only at night, there may be no effect.