Raise it, or get a plumber to do so
Yes, a sewage drain pipe can be too large.
Probably the wrong size flange. Is the flange a toilet collar? If so the pipe is probably 4 inch.
Install a ty fitting on the stack in the basement then run it to the sink. Put it low enough to account for a fall in the pipe, to allow proper drainage. Be sure to install a waste and vent pipe to code for both fixtures.
Waste water will run away too quickly to properly carry away any solids in drain pipe, eventually causing a clog.
Ingested drain cleaner is a corrossive substance do not induce vomiting.Acidity is high.
1 1/2" or 2". Depends on your pipe size on the trap arm. 1 1/4" is too small for a kitchen sink and not allowed.
First thing you should do is call a plumber. Drain lines are not supposed to leak. Your drain line is clogged and needs to be snaked out and the leaking pipe repaired.
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.
Extend the overflow pipe underground to a point where it is above ground.
I have that problem too. The smell comes from mildew growing on the sides of the drain pipe above the waterline in the trap. It's rather dirty in there below the drain plug, and you never really clean it. When you turn on the water, it disturbs the mildew and releases bad smells. The water trap in the u-turn under the sink keeps sewer smells out but this is before that. The solution is to grab a bottle of bathroom mildew remover and spray it heavily around the drain plug to saturate all sides of the drain pipe. Try this once every few months to keep your bathroom smelling fresh. Don't forget the shower and bathtub drains aswell, and kitchen disposal too if you have a problem there.
3/4 inch would be too small for anything but just liquids. As long as there is a trap under the sink, there shouldn't be a health hazzard. You will have to be a little creative in connecting the sink drain to 3/4 but possible.
There is slight blockage in the pipe probably further down, the pipe needs a good cleaning througout the length of the pipe, preferably with a wire/brush type cleaner, checmical cleaners or drain type cleaners may be too weak to clean it due to the length of the pipe.