Actually I'd think the reverse on leathality radius of a depth charge would be true at maximum depth. The hull is under much greater strain at depth so a blast that might only dent it at shallow depth would be more likely to collapse it at extreme depth. Also note that a small leak at shallow depth was fixable, at maximum depth the slightest breach of hull integrity was instantly fatal.
And as to the question of U boats lost purely to crushing, I doubt anyone would know, as no one has ever survived a submarine implosion to my knowledge, and only the crew would know for sure.
4) the weakest spot stucturally speaking in most submarines is the junction of the conning tower to the main hull where the curve of the hull is interrupted.
5) The type VIIc/41 U boats had a thicker pressure hull to increase dive depth compared to the stndard type VII.