Another one of those things that needed sorting was that there was no heat. After some search on benzworld, I figured it was either the monovalve or some sort of secondary water pump. The monovalve seemed like the easier one to look at first, so I ordered a monovalve repair kit from autopartsway Canada, cost $70. I wanted to order it from Autohausaz because it was only $30 but because of Authausaz stupid shipping policies, the final cost wasn't worth it.
The procedure for changing the monovalve is a lot simpler than what the workshop manual suggests. You don't need to drain the coolant or anything like that. I had to remove that little computer first to get more access, but that was the extent of the complexities.
Even though at this point I was still not 100% sure that a repaired monovalve would sort out the heat, the rubber seal on the old monovale had ruptured and clearly needed replacement.
Be careful when removing or putting back the four screws into assembly. Its impossible to get those screws back if you drop them. I managed to lose one, thankfully, the three remaining screws were enough to hold everything in place.
And yes, wonderful warm heat is working again!