Inference follow up after #5129

While #5004 is being consolidated into #5129 this issue collects some left overs in this area.

Generally, all code tagged `NON-JLS` is worth revisiting, but in this ticket I will focus on aspects that are not coordinated with javac.

Review issues #5004 and #5129 first to understand the inference work being consolidated. Then inspect code tagged NON-JLS and identify the aspects not coordinated with javac; the issue provides no named files, tests, or completion criteria beyond collecting those leftovers.

Written by the indexing model from the issue text.