1. No. He was only dead 80% of the time
2. The undines wanted to stop the flood to extend their lives and reign

>>23627204
1. he didn't drown in Gyoll as a kid? That seems implied
2. they live in the sea, so doesn't the flood extend their territory, and why do they do they seem to accept it at the end?

>>23627249
1. Matter of opinion. Consensus is that when he dies they just revive him but on the ship he dies for real and continues his journey as a man/ghost.
2. Undines need servants and victims (food?).

>>23627282
The skull he finds of the "boy who died in his place" I took to be the original Severian, but then the question would be what leant him the energy to sustain his duplicate form from then onward.

Also, Baldanders I guess was some lower servant or creation of the Hierodules, who either rebelled, or appeared to rebel in order to push Severian along in his journey?
He was gathering data on earth, which is what the experiments were about?

>>23627358
Baldanders wanted to be Urth's representative and was collecting as much knowledge as his body could contain. But he discovered he would lose when the Hierodules bowed and praised Severian. So he battled Severian despite already knowing the outcome.

>>23627302
Maybe, maybe not.
simple answer is "the hierodules wanted it that way" at least for the original tetralogy