Probably the wrong size flange. Is the flange a toilet collar? If so the pipe is probably 4 inch.
1% = 1/100 means 1 inch drop over 100 inches of drain pipe that is slightly more than 8 feet 1 in drop over 8 ft 4 in
Wouldn't recommend it as things may back up in the drain line. 4" line is the norm ... you want to get rid of waste, not keep it around longer.
With large diameters like that you will most probably need to insert a weld-on 12 inch - 10 inch reducer at the point where the reduction is required. The pipe and reducer perimeter edges should be bevelled, say 45 degrees, for good weld penetration especially if the pipe conveys fluid under pressure.
The volume of a 12 inch x 12 inch pipe is: 1,360 cubic inches
1/8" per foot.
Probably the wrong size flange. Is the flange a toilet collar? If so the pipe is probably 4 inch.
1% = 1/100 means 1 inch drop over 100 inches of drain pipe that is slightly more than 8 feet 1 in drop over 8 ft 4 in
1-1/2 inch drain for above grade and 2 inch drain for below grade. Most UPC-based codes (IAPMO) require 2 inch above and below grade.
100
Depending on the type of drain pipe referred to, "Yes", it can have too much drop or "fall". Specifically sewer drains, as too much drop allows the liquids to leave the solid material behind, causing a blockage. The water outruns the solids.
a 3 inch pipe
12-13 inches to the center of the drain.... make sure your drain pipe is 1/4 inch drop per foot. also use the sweeping y drain if possible. leave lots of room for the flange. mount the toilet after the floor is in.
yes
your drain has a restriction in line and needs to be snaked
Absolutely NOT you 2" minimum
You just replace it. How depends on what type of pipe. Metal pipe, the trap is usually held on with nuts that connect the trap to the sink and the main pipe. On PVC or plastic, it may be glued in place. Rarely is all the drain glued. The piece going up to the sink should have a slip joint right above the trap with a nut. There should be enough of this tube in the trap to allow you to cut the trap out and glue in another one. Cut the trap off the drain line as close to the trap as possible. The drain should be straight at this point. Glue a new one in and reconnect to the sink. The trap will drop down 1/2 to 3/4 of an inch, but there should still be enough pipe to connect. If there is not, an extension is available. Without seeing it, there is no way to give more specific directions.