Thanks for your questions.

1. The TP4056 is charging the batteries at 1A. Without any losses due to heat it would take(21Ah/1A=21h) 21 hours. But nothing is ideal and also at around 4.2V the Tp4056 is going into the constant Voltage mode which means it is just topping up the pack with less then 1A charging current. So it takes around 25 hours.
2. I used the free website: https://easyeda.com/ to create the electrical diagram.
3. I haven't planned to do it but it would be a good idea. Maybe I do it in the future.