A320 Descent Management - VVCR glide slope from above 

You mentioned "it's a good idea to level off". That statement is not correct. The correct statement is "If you need to descent but can't, you need to slow down". The level off is self evident because you can't descent, so naturally you have to level off. As for why you have to slow down, good question. The answer is two fold. Technically you are adding energy to the aircraft in the form of speed if you don't slow down. If you are high on energy, you don't want to make it worse by adding even more energy. You have to disappate this extra energy with speed brakes later on. So first you were adding power to keep the speed up, then later you need speed brakes. That is a waste of fuel and in the worst scenario you have to do a go around because you are too fast on the approach. In practical terms, you should slow down if you need to descent but can't because if you do so, you can dive down later and let the speed increase, getting back on your vertical path, something you can't do if you were already fast.

Yes, but make sure the speed bug is below the current speed, don't select speed brakes with Flap Full, and don't forget to arm it again if previously armed. Gear down has more drag and less to worry about in that situation though.