Hi there Devildog,
I had some poorly designed venting in a house a while back. The kitchen sink drain was too far from the vent stack. So the garbage disposal never did work right.
I don't have first-hand experience 'snaking' from the roof, but I've seen it done. I would imagine that this would remove any stuck birds, however.
As for the toilet in question, I guess it may be possible that there is some kind of clog that plunging isn't clearing. It's probably easier to try a toilet auger than to reengineer vent pipes. Just in case there's a mass of hair or something that plunging isn't working on. I would think that if it's a clog of some sort, it would get progressively worse--not stay about the same for 2 years.
But as we all know, plumbing can take on a life of its own!
Best of luck,