Toilet gurgling when another one is flushing: 6-way joint
Move the clean out trap up to where the right toilet is,right toilet to the clean out. 

This will stop cross flow from too large systems.

This should reduce cross flush and vent clogging, I think.

This looks to be the easierest think to help reduce your issues but I dont know.

That 90 in the waste after you puzzle nut can't be helping either... how far does it travel before the main stack? 

Keep in mind that thing is grandfathered, touch one bit of it and it's not anymore.

Not a plumber, just a dude with a internet connection so take my advice with a grain of salt.