Poop in my sump pump crock, how do I fix this?

This is a problem we've had for a while, but up until now it happened every blue moon. Now it's gotten to be quite the disgusting nuisance. Let me clarify, not actual solid waste is getting in, but seepage. Smelly smelly seepage.

So here's how my basement is set up. We have an old house which had a sump pump installed many years after construction was completed. From the crock there is a line running under the floor towards the sewer line, I have this plugged with a test plug. The sewer line running from the second story to the basement is about five feet away from the crock and drains out to the street. There is also a toilet about five feet away from the sewer line, however I don't think the toilet is contributing to the problem as I haven't used it in months. Picture if you will an "L" shaped pattern where the sump pump crock is at one end, the toilet is at the other, and the sewer line is the junction.

Here's the problem as I see it:

I used to think the line running from the crock towards the sewer line was to ferry water out of the sump pump crock in the event of sump pump failure. I now think it is meant to be a vent for the toilet in the basement because the toilet won't flush properly now that the line is stopped up with a test plug.

The sump pump crock sits several inches lower in the floor than the sewer line. At least this is my guess since it's plain to see by looking at the floor levels where both of them meet the floor (although I suppose the sewer line could travel several inches below the concrete before heading out to the street.). Therefore it's my thinking that the line running from the crock towards the sewer line slopes down towards the crock.

I removed the test plug today to find lots and lots of solid waste backed up behind it. It didn't smell great. Here's how I think it's ending up there. I make my business in the second story bathroom, flush the toilet, it falls two floors, hits the bottom of the sewer line, goes splat, and some of it is getting washed downhill towards my sump pump crock. Does this sound plausible?

So here's how I'd like to fix this, maybe some of you can tell me if and how this can be done. I'd just like to seal off that line running from my crock towards my sewer line and I'd like to seal it up inside the pipe itself because the test plug is doing nothing. Doing this will render my basement toilet unusable, but I'm ok with that since I never use it anyway. So would it be possible to do something like this without jackhammering my basement floor? If so what should I use? I have a plumber coming out Thursday morning to take a look at it so maybe he can offer some advice.

Sorry about the longwindedness and thanks in advance!

Either your diagnosis is completely incorrect, or your installation is about as bad/illegal as it is possible to do it. There should be NO connection between the sanitary sewer and the "crock", other than a pump discharge pipe. IT would also be impossible to connect a "vent" to the crock and thus there is no proper way to do it, period.