The danger point for lipo is 3v per cell. However, when the cells go out of ballance, that is a good indicator that you are over discharging the pack beyond what is best for it. You have not hurt the pack, but it is still a good idea to try not to hit the LV cutoff if possible.
Your efficiency is phenominal! My recumbent pulls 380 watts at 20mph. You have a very efficient setup there.
You should be good for 25 miles at 20mph without pedalling considering your efficiency.
You should charge it up, go for a 10 mile ride without pedalling (other than to start out) and see exactly how many WH you pulled and you can calculate your WH per mile. I am guessing it will be under 12wh per mile at an average speed of 20mph.
1% of the world's population can think "Outside the box". The rest are firmly stuck within the box. Where are you?