1 - Here is a time consuming way to confirm if the add-on is for wastewater. [BTW, I've done this, but for other reasons.] Check your manual (or google) to determine how much wastewater storage you should have, i.e. 30 gallons, 40 gallons. Then you can get a water meter (https://www.amazon.com/gp/product/B0...8D2FH3WRX&th=1
) OR you can do it the painful way like I did. Pour one gallon at a time into the toilet until the you reach the printed amount. If you have more room in the tank you know the PO increased the size of the wastewater storage tank. Now that you have gotten this for, may as well go head and fill the tank until it will hold no more. Then you'll know the total amount of wastewater (in gallons) that you have available.
2 - From looking at the pictures you provided it appears that the PO has make you a easy access way to flush out the wastewater tank. Hence the copper pipe that is just to the left of the drain pipe.
I "assume" there is some reason you can not contact the PO to inquire as to what or why he/she made this add on.