> If you send a console command to move the cursor or do anything else to it, the simple answer is to make that a barrier between glyphs. If you output a combining character after doing that, the combining character goes into the next cell all by itself.

Maybe. I'd be concerned about what effects that might have on applications which need to be able to display multiple streams of output at once, like tmux.

> Being the last character on a line isn't really any different from being in the middle of a line

You might be surprised. After printing the last character on a line, the cursor goes into a weird "wrapnext" state, where the cursor stays at the end of the line, but will wrap to the next line (potentially triggering scrolling) when the next character is output. This state is highly ephemeral; it can't be entered through control sequences, even by DECSC/DECRC.