Keeping batteries charged reduces their life. How much exactly, I don’t know. But as a rule of thumb, I try not to store my batteries full for more than a day. If I have have charged them, I see it as my duty to fly them.
Charging batteries with more than 1C also reduces their life. Again, I don’t know how much, but I try to always plan ahead and charge at 1C. But if I’d miss out on flight time, I charge them at 2C. That’s half on hour. I wait half an hour and answer some questions in the meantime
Temperature reduces battery life time. At the same time, temperature increases performance. I don’t cool my batteries, but I take care not to expose them to heat (car, sun). When I go fly, I store full packs in my left pants pocket to keep them warm. Empty ones go to the right pocket.
Finally, don’t discharge them too much! Anything below 3.6 resting (!) voltage is too low. If you punch the throttle on a 3.6V battery, you can easily drop the voltage below 3.0V, which is not good for the battery and the electronics usually don’t like it either.
Parallel charging is OK, but poses some risk. If you connect batteries, they have to be very close in voltage, or you get compensation currents that can exceed the charge rates. Also, calculate charge rate properly!
If you do it right, parallel charging is perfectly OK.