We should be clear about what the issue is here...
The "Confirm when pasting" dialog box only pops up when you paste (as you pointed out) N or more lines-- in your case, 1 or more.
However, your initial bug report says that "x", specifically "without a newline" does not pop up this dialog-- it shouldn't. It's not "1 or more lines". You need a newline character for mIRC to consider this a "line"-- otherwise it will just copy into the editbox as normal.
Are you only having this with less than 1 line of text text that does not end in a newline? If so, this is normal expected behaviour.
Sidenote: keyboard shortcuts not working in this confirm dialog box seems intentional to me. It would subvert the entire feature if the confirm box was ok'd by pressing more keys, since the whole purpose of the feature is to ensure the keys aren't being pressed accidentally. I can imagine many people pressing the key combination <Ctrl+V><Space>, or <Ctrl+V><Enter> in an immediate fashion, and not to intentionally remove the dialog box, but just because inserting a space or a newline after pasted text is extremely common. Forcing the mouse ensures that you are really confirming this action.
