Is it problematic if an apology can't contain much-needed nuance?
Absolutely yes. You're assuming that an unqualified apology, accepting the blanket of charges being levelled, is the only productive recourse. IMO that is the most damaging thing he could have done for the party, as well as himself. Just because some would have felt nice about it is irrelevant at this juncture. To put it in terms you've used, in order to be seen to capitulate and remediate regardless of personal belief of the reality and complecity of the issue, to oversimplify and apologise unconditionally would be insincere.
The correct thing to do at the party level
I don't really understand why you and he find this so difficult.
"The correct thing to do" – according to who? The party is already fucked from heel dragging, egoistic, centrist obstructionists and the gratuitous public self flagulation under Starmer, regardless of Corbyn's words or actions. And also fuck Starmer. I think there is your answer.
The only thing a leader needs to do to sound statesmanlike. The more nuance you add, the more weaselly it sounds.
1) he is no longer the leader, 2) details are important.