In the log window of version 19.1 there is a "Use Message for Commit" command.

Should it apply to the whole file or just for new characters?

Right now, we enhanced the interactive rebase to highlight equal named commits, and provide options to squash them.

What is still missing when using Ctrl+P and Ctrl+Shift+P?

In version 19.1 one can show/hide merged commits if "Follow only first parent" is selected.