If you’re not applying throttle or brake, then the VESC will let the motor just freeroll. Although if you go very fast for example downhill without motor power, if the motor reaches high enough speed it might start pushing energy back into the battery without brakes being applied.
In theory yes, but not really in practice, except for a particular scenario.
When you’re braking and energy is pumped back to the battery it’s voltage will raise, how much the voltage raises is dependent on the braking current, motor speed and the battery’s ability to absorb the incoming energy (this depends on battery’s current rating and capacity).
What you want to avoid is any of your cells in your battery going over 4.2V, but this isn’t really a problem during a ride and you have discharged the battery already so it’s cells are not near the 4.2V, how much lower will they be depends on how much energy you have used, but generally the cells will be able to absorb the energy without their voltage raising too much.
The only scenario where this really might happen, is that you have charged your battery to the top and you immediately have a long downhill on the start of your ride, which you need to use brakes on. With the battery being full, it’s cells are very near that 4.2V full point and anymore energy into the battery will raise the cell voltages over that limit. This might drastically shorten the life of the battery or if it’s even more, then the cell might experience a thermal runaway (aka, burst into flames).
I would recommend having some brakes on the board, if you’re not sure, just set the negative braking currents to something low. So knowing these rules and you knowing your riding route, if you charge full, avoid strong braking in the beginning or alternatively, don’t charge completely full, but leave a little “breathing room”.