I've seen drywall dust/mud/primer etc left under carpeting just like your pic. 

Just go to your local DIY supply stores and take notice of how many different types of sealant or spackle mud there are.

The bathroom for 188-2 is in drywall and mud now it was originally a hallway leading to the unit but by moving the entrance and shifting a few walls I was able to create a new bathroom with side by side washer and dryer.

If it were me, I'd pass on this one, because if you're planning to buy more properites in the future, this may put your debt to income in an undesirable range for future loans. 

Our tenant likes it because it gives her little ones a spot to run in and use the bathroom when outside with friends instead of tracking mud through the house.

But you can buy a new 3-2-2 for 240k...garden home, further out but nicer neighborhood, same school district (But Hoa or mud taxes for sure).

HOME OWNERS IS THE SAME THING, JUST BIGGER NUMBERS WHEN THEY PAY CLAIMS.I hope that's as clear as mud now.

I’m looking for things such as blogs, books, Youtube channels, etc.I know a few basic things such as painting, drywalling, and mudding, but I would like to learn a lot more.

Finally, one plumber told me pipes might have cracked/broken under the ground, as his camera got stopped 50 feet in and came back with mud.....He suggested to either open the floor and fix it thoroughly , or build a pipe on the outside wall and re-route kitchen to the main pipe directly...Both options sounds horrible to me....Any advice please?

Clear as mud!