>>3905702The way the meter is behaving (sometimes not working at all, then suddenly working), sounds like intermittent contact issue. Maybe related to the fact that the prism is loose on one side.
Make sure you clean all contacts, use the correct battery, and see if something is bent or in the area that is loose is preventing it from making proper contact.
About the things on the mirror, it could be desilvering (nothing to do there), or pieces of foam from the lightseals and the mirror dumper disintegrating into a sticky goo and getting stuck on the mirror. See if you can carefully remove it.
About the focusing screen, clean with warm (not scorching hot) water, and some soap.
Don't rub it with anything, especially any kind of fabric or paper, unless you want tons of lint on it.
Don't use alcohol or other solvents cause it'll get destroyed (cloudy).
Just warm water, soap, and gentle rubbing with your fingers. Let it dry somewhere (no wiping it dry).
On the issue of lens dust, it's not important and doesn't affect photos in the slightest unless there's a huge buildup.
You can't avoid it, the lenses aren't airtight, and with the focusing movement, they act like a pump (even more so when zooming with a zoom lens), sucking in some dust. The more you use the lens, and the more dusty the environment (same for bags, etc.), the more dust you will get.
No big deal though, you can always, after many years and *if* there's significant dust buildup, unscrew/disassemble the lens and wipe it out.
Usually you don't even need a full disassembly, just a couple rings to separate the lens into 2-3 optical blocks and wipe the exposed surfaces.
