Lazygit maintainer here: I've found myself in your shoes quite a bit (without the commit signing part) and a few weeks ago I put up a draft PR where if a file is selected, it highlights the commits that touch that file. Typically you want to amend the most recent commit that changed the file and typically that commit is visible without needing to scroll. But I haven't spent much time thinking about what the ideal UX is, how to activate it, etc.

PR: https://github.com/jesseduffield/lazygit/pull/2654