AM should be along soon to give more detailed help but in the meantime you get me.
The guage died on the one I used at work but by then I didnt care as I no longer used it as a reference.
I suppose at this stage it might help you learn the ins and outs of this particular machine.
Im afraid the best way to clean the head will be to at least pull the screen off so you can get in and give it a scrub.
Just make sure you dont force anything.
From memory, the bolt is 12mm but check first.
The collar does sound very worn and a replacement isnt cheap.
AM may have some ideas.
The filter sounds like it needs replacing, Im not sure if you can recharge them like in my machine. AM?