Tua is second person. Sua is second or third person.
Actually, we use tu/teu/tua (2nd person) in Brazil, but it's used only on the Southern Region of Brazil, which includes the states Paraná, Santa Catarina and Rio Grande do Sul. Pretty much everywhere else in Brazil você/seu/sua (3rd person) is used. I believe in Portugal they use the 2nd person regularly as well. About the other countries that speak Portuguese though, I have no idea.
The possessives must agree in gender and number with the thing that is possessed, not with who possesses it.
seu - singular masculine
sua - singular feminine
seus - plural masculine
suas - plural feminine
It is always "sua salada" because "salada" is feminine. It has nothing to do with the salad belonging to a man or a woman.
It can. How was the question presented to you?