“Same problem, different entry point” is exactly the point of duplicates. If two questions are about different instances of the same problem, and it's obvious how to adapt solutions for the one into solutions for the other, then the questions are duplicates. In this case, the only difference between How to distinguish C-m from RETURN? and How to distinguish C-i from TAB? and How to bind C-i as different from TAB? is whether they're about C-m/RET or C-i/TAB. So they are duplicates (all three of them).

I also want people searching for C-m issues to have as nice a landing page as people searching for C-i issues.

Duplicates don't hinder that. Questions that are closed as duplicates are still available as search fodder.

In this case, the phenomenon is the same but there are a few specifics that are not obvious, namely the correspondence between RET, TAB, ESC, etc. and the Ctrl keychords. The best solution is to add a sentence to the answers providing this correspondence.

“Same problem, different entry point” is exactly the point of duplicates. If two questions are about different instances of the same problem, and it's obvious how to adapt solutions for the one into solutions for the other, then the questions are duplicates. In this case, the only difference between How to distinguish C-m from RETURN? and How to distinguish C-i from TAB? and How to bind C-i as different from TAB? is whether they're about C-m/RET or C-i/TAB. So they are duplicates (all three of them).

I also want people searching for C-m issues to have as nice a landing page as people searching for C-i issues.

Duplicates don't hinder that. Questions that are closed as duplicates are still available as search fodder.

In this case, the phenomenon is the same but there are a few specifics that are not obvious, namely the correspondence between RET, TAB, ESC, etc. and the Ctrl keychords. The best solution is to add a sentence to the answers providing this correspondence.