We like the way you think! Yes, Rust-Oleum Varathane Oil-Based Ultimate Polyurethane Spray can be used on previously finished surfaces and on bare wood. For the best results, we recommend applying several light coats a few minutes apart. You also want to ensure that you are applying in a well-ventilated space. - Rust-Oleum Product Support 3
3 coats should get you a hard and smooth finish. You probably need to spray it on thicker.
Yes I would use a sealer on soft woods and wood that is prone to blotching.
No I Would't it isn't made to walk on
I do not recommend it for floor repair hope this helps