Why isn't daha necessary here?
the ablative ending -den/-dan gives us a clue about that, so in daily speaking we may omit daha.
Does this apply to other s̶u̶p̶e̶r̶l̶a̶t̶i̶v̶e̶s̶ comparatives as well? So is it also valid to say "Benim gelirim seninkinden buyuk"?
Yes your sentence is also correct but it has nothing to do with superlatives. You can't compare two things in the superlative form.
Benim en gelirim en fazla. -> My income is the most.
Ah yes sorry, wrong terminology.
You can use it, the meaning does not change. It is accepted.
Why is " Benim maaşım seninkinden daha düşüktür" is not acceptable?