One Step Forward, Two Back?

Today was a typical old-home-owner type of day. First the good news. My brother-in-law shored up the termite damage, as seen below. He did a great job and it certainly looks much better. We did not insulate yet as this area will hold the washer and dryer, so we need plumbing and electrical. Then we will insulate everything and seal it up. We are leaning towards bead board here, so we will not plaster but throw up sheetrock instead.



















My father-in-law is pretty good with plumbing, so we moved the drain from one side of the kitchen to another (no pics, will post later). So while that saved me some good money, we move onto the not-so-good, potentially-bad-stuff from today.

I know that setbacks are guaranteed, but I was hoping not to find any BIG ones. Towards the end of the day, my father-in-law and brother-in-law said, "We heard some gurgling in the toilet that time forgot." It was happening every time someone flushed the toilet two floors up. No sooner did he say that did the water start bubbling over from the toilet onto the 115 year old floor. It looked like poop and smelled even worse. It was poop. Not good. I suspect it was doing that for a while without overflow, but with 6 people using the toilet today, it pushed it overboard.

Needless to say, something is wrong. Since I am not a plumber, I deferred to other, smarter people. They say it is likely a clog in the main. However, I just had the main scoped prior to purchase and it was clean as a whistle. So, can there be a clog from the upstairs toilet to where the scope went in? Where would the backup be? I was told that water will back up to the lowest point, which is the 50's shitter in question and seen here. There is also some water in the tub, too.

I'm sure this is somewhat common. I'd rather not spend a fortune if I can fix this ourselves but my time is also needed in 1,024 other areas so I need to be realistic if this is beyond me. Any thoughts are appreciated... Or, if you are in the area, I pay in food and booze.