Even in the case of
"Hello
a."
I'm still pasting only one line, because mIRC does the default action of /say on the first line and the second line is left in the editbox because it doesn't end in a linechange.
That's not true. It will send both lines immediately in this situation (which I think is good behavior, since it seems like you would want that to happen in cases of pasting multiple lines).
The fix for the single-line issue is simple as I described already here:
https://forums.mirc.com/ubbthreads.php/topics/238844/Re_Pasting_text_with_line_chan#Post238844
