On Q1, a bit higher than 5 cm is fine as long as the connector sits on a flat part of the tank wall. If the hole lands on the curve the washer will not seal properly and it will weep. On Q2, yes, cut the pipe back and remake the joint with a compression coupler, just make sure the cut is square and the pipe end is clean. Once it is full, leave it a while and run the back of your hand around the connector to feel for any dampness before you box it in.
Great, thanks for this.
