@Bas-de-Reuver Notepad++'s regular expression search/replace has a couple of things that you and others may find useful.
\R matches any style of newline such as CR, LF, and CRLF. While \R does not work in the replacement part it’s still useful.
You can remove blank lines using ^\R+
You can remove spurious blank lines using ^\R\K\R+
The \K says to keep everything to the left meaning it will keep one blank line while removing spurious blank lines.
In both cases it’s a regular expression search/replace with the replace part being nothing or blank. This also means we don’t care about the current end of line format as \R matches any of them and \K ends up keeping whatever end-of-line style is in use.
It’s not clear from the OP’s example if they desired to remove the blank lines and then sort the resulting list or if they desired to have some sort of combining, merging, or interleaving of the lines that may or may not be sorted. As the OP then wanted to delete the thread it will likely forever remain a mystery.