Re: Weird situation
Water levels dropping in the toilet are usually a blocked vent issue. As water travels down the drain, it needs to be able to pull air in behind it, otherwise it will be pulled through the traps of the fixtures, resulting in those traps - in this case the toilet - being drained.
Water spouting out of the roof vent would be a backed up drain somewhere, though if it's spewing out a roof vent, then you should have noticeable backing up in the shower, tub, and possibly toilet overflow first.
At any rate, what you describe all sounds like drain problems. I would either just rooter everything out or have a plumbing company come in and run a camera down the system to see if there is a blockage, roots, or collapsed drain line.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!