No daha needed?
Nope, most of the time daha is optional actually :) Both ways are commonly used in speech and writing.
I've read that daha in comparisons is more like "even" - optional but makes the comparison stronger.
So "Benim gelirim seninkinden daha az" would be "My income is even less than yours".
Is it "senin-ki-n-den", where " "ki"means "that is" senin?
Something like that -- you have senin "your ..." versus seninki "yours" (which you could understand as "the one that is your ...").
What dies thr ''in '' do here ?
The word is "seninki-n-den".
the n is for merging the extras
"My income is smaller than yours" - reported as I believe it to be correct as well.