You want the drain pipe for a kitchen sink at least 18" above the floor. You do not want it any closer or it might be kicked, or damaged during building or installing the faucet pieces.
Yes, be sure to pipe floor drain line properly into water closet piping.
yes.
The toilet flange is usually above the floor level. You would have to cut it off the drain pipe and cap it. How you do this depends on what type of pipe the drain is made of. Even if the drain should be below the floor level, you have to seal the drain or sewer gas will come up from the drain.
Raise it, or get a plumber to do so
another clog, further down the drain pipe. You need a drain snake.
It will dry just fine... after you remove the flooring...
Inside pipe cutter
To drain the water that is in the pipe or spout..
12" center of waste to the floor
drain or gully may have another inlet coming into same pipe via a junction under the floor
The drain is either not high enough or partly clogged. It should be about 36 inches up off floor. Adding a bit of ABS pipe is easy, or run a thin snake down and unclog it.
Somewhere down the pipe from where the washer and kitchen drain combine, there is a blockage. You will have to find where these join and look further outward for a 'cleanout' plug, then fit a snake down this plughole.