Behind the toilet? Wow. I never knew he was so talented. 
We used to have a tree in the front of our house that would send roots into the sewer line and back things up. Since our toilet is furthest from the sewer, it would back up first. Not very pleasant (like it backing up into the shower) but it didn’t affect the seal ring. I’ve replaced a few of those also, and they are the cheapest replacement part, outside of screws, in the hardware store.
Did your toilet flush when you had this problem? Hard to believe it did.
We rate our projects on a sliding scale, including number of runs to the hardware store, and number and kind of tools required. Hand tools are low rated, electric tools higher, anything with flame higher yet, and pneumatic (air or hydraulic) getting bonus points.
As someone advised up thread, best not to begin any project that might affect habitability on Sunday or a holiday. Monday AM leaves time for disassembly, driving madly all around town, considerable cursing, then overnight air of some critical part now found to be locally unavailable. (Really, I’ve had this happen, though not commonly by any means.)
We had a toilet leaking about a year ago. It turned out to be a source of the mold on our wallpaper out in the next room! The water was seeping into the wall. Was a fairly major job, with the plumber having to move the toilet to get at the problem. Luckily for us, plumbers here get paid about the same as burger flippers in the states, so I think it was maybe $60 for the whole thing. And that was expensive for plumbers here, so it was definitely a major job.
Yeah, but the human leaked first.
It would halfway flush. If you tried flushing it again it would fill the bowel but not overflow. Using a plunger made it go down but it never would clear. Finally it just came pouring out the bottom just as I was packing my bags to get on an overseas flight. Yeah, bummer. 
Okay, so I, um, haven’t done anything about it, of course. Now, it howls like a wolf when we flush it. It was doing that a while ago so I got a repair kit and never repaired it (a pattern develops?) and it stopped on its own. Could it be the same problem? In other words, will replacing whatever parts are in that kit fix my leak also? Or is this just the Great Conjunction of plumbing problems?
They could be relatesd, or they could NOT be related. The only way to know will be to diagnose your leak problem, correct it, and see if that also corrects the howling problem.
In my experience, that howling sound is the inlet valve.
And in my experience, it makes that sound right up until the moment it fails.
It turned out that one of the smartest things I ever did was to buy a house only five blocks from a big hardware store. I’ve learned to always bring the replaced part with me, and to buy something else just in case.
Geez, the problem is spreading. Last Friday night, the wife noticed a long wet stain along where the wall meets the roof. Turns out the unit above ours has developed that problem.