@Mansoor Fazel, is this crack horizontal or vertical?

IF you have a cast iron stack and galvanized horizontal runs to the fixtures the MOST likely place of plugging is the place it joins together right at the stack. 

Then Scott you may have a "breached" or open vent pipe in the stud wall serving that fixture.Sometimes old galvanized vent pipes rust and go rotten over time leaking sewer gasOnly solution is to open the stud wall and inspect vent pipe,the leak or hole could be anywhere along the linear footage of said vent horizontal or verticalHope this helpsDD

Personally, I prefer to enter my data vertically (on a form) vs. horizontally (directly on excel).

I used a handheld belt sander with a 40 grit belt to quickly get the high points knocked off then 3-4 coats of joint compound to skim coat (alternate vertical and horizontal strokes), then re spray a light texture to make it look normal.

Does that mean I can't get a Horizontal Property Regime applied and create two houses on my lot?

Depending on how bad is the foundation state (how many inches you have to correct), is very possible the sewer line will disconnect/break in the horizontal portions.

If so, are the cracks 'horizontal' (go from left to right), or are they 'vertical' (up and down)?

Also maybe changing the vinyl cover on the horizontal support beam.

It appears the slab is split horizontally and is causing some minor shifting and cracking of walls.