I agree with you cent percent BUT why did then author choose to use "not so much a retrospective as a _________;".. it appears as if the author is looking for a word what is not praiseworthy. Please correct my under standing.

Paen and euology . Both the words mean the same.

A proper retrospective should include all the details of his early work and everything up to his latest accomplishments.

But we are told that the artist's early work was glossed over, which means it was concealed and disguised because does not look good. We are also told that any evidence of his ultimate dissolution is absent entirely. So his final dissolution as an artist is also absent. So basically all the negatives have been glossed over or eliminated.

So only the positives remain.

Tirade and philippic are denunciations. So we eliminate them, as the exhibition presents positives and not negatives which need to be denounced.
Panacea is a cure for all ills and crescendo is the highest pitch of his work. But there is no information in the sentence to suggest this exhibition was panacea to something. Neither does it present the highest pitch of his work.

We are left with paean and eulogy which are both praises and fit well. And what we would expect if the exhibition only contained positives.

"not so much a retrospective as a _________; " means it looked more like a __________ than a retrospective. A rather roundabout way of saying it was "not a retrospective, but a ________".

But it also means there was something of a retrospective in it, but more of a ___________; Thats one possible sense. Though it could also mean it was wholly a _________, not a retrospective at all.

Replace the blank with paean/eulogy.