surprised helix doesn't support the . key - is that incompatible with its interaction model somehow?

Yes, its model is that you make a selection and then act on it. After the action happens there's nothing selected. The editor can't infer your intent in making the selection, so there's no reasonable notion of repeating the last edit. You can separately repeat the last motion or last action, but not the whole edit---which of course isn't as useful.

Helix would _ideally_ like you to tell it ahead-of-time everywhere you're going to make an edit by selecting all the edit points with multiple cursors, without a great option to fire edits on the fly. Ironically I found this worse for visual feedback when making larger edits---where it's most needed---because there are usually edits happening off screen, instead of only at the point of the cursor (where the latter can be easily repeated).

It ends up feeling less interactive and more like I'm using a batch editor due to this, which (I think?) was not its intention.