If I understand you correctly, you do not like the set "‒" separator between lines in certain messages with information, but at the same time, in all other windows and messages, it must be present to separate lines of text.
In this case, with all the capabilities of the user when working with the client, I see only one solution, this is to disable the separator in the mIRC options: Other -> Line separator -> (leave the field blank)

And for the necessary places and windows, create text event handlers in the scripts where you will need to add a line separator. For instance:
Code
on *:TEXT:*:#: .echo $chan $chr(45)
on *:TEXT:*:?: .echo $nick $chr(160)
on *:NOTICE:*:?: .echo $chan $chr(8210)

And so on for all types of messages...

Perhaps this option will suit you and will help you customize the display of messages in the desired form.

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