So, I have finished the story modes of both Single Player and Multiplayer.
I must commend the following players for assisting me in getting to the ending of multiplayer:
Timboman (yes, he's still alive out there
Tesseract (ditto on that part)
Erebus and I started it off, got to about part 2 and called it good, Timboman and I played til about part 4, then I replayed with ER, got him up to speed, then finished it all off with Timboman to the ending. I won't spoil any of it for Erebus or Tesseract, but I will assist them when they want to play XD
Tesseract and I went in while he had not played the multiplayer, so I got the Professor Portal achievement, which is described as the hardest to get.
My achievement status is 39 out of 50. The majority of them appear to be singleplayer achievements now, so it's just a matter of playing through to where those are and be done with it. I sadly cannot see a way to replay just a single level, unless I use the console commands (I might just have to
This will be fun getting to 100%. And I have a feeling there'll be more chambers down the line.