1. No. Immemorable = that can't be remembered, or that's not
worth remembering. The word exists in theory, but it's not used. There are
more idiomatic ways of expressing the idea, e.g.
‘Playing With Fire’ is an entirely
forgettable film that’s unlikely to become one of your
children’s treasured classics (thejamreport)
(No-one would say „immemorable“ here.)
2. No sooner than John sold his house the one he
was hoping to buy fell through.
- 2.1 No sooner had John sold his house than the one he was hoping to buy
fell through.
- 2.2 No sooner did John sell his house than the one he was hoping to buy
fell through.
- 2.3 John had (only) just sold his house when the one he was hoping to
buy fell through.
2.1 means the same as 2.2. The past perfect („… had John sold …“)
isn't necessary because the the time sequence (first he sold his house, then the
sale of the other house fell through) is evident from the „no sooner“
construction, and doesn't need to be conveyed by using the past perfect tense.
2.3 is the most idiomatic version in spoken conversational style.