Feif Un: That’s not respect. That’s a separate issue. The SH”A (maybe Rama) at the end of Siman 128 says that it’s Assur to use a Kohen for anything. A Kohen can be Mochel, and presumably they are, which is why most people are not Makpid on asking Kohanim for favors. Still, the Ikkar Halachah is that way. There is a separate Halachah that a Kohen should be served first at meals (Litol Manah Yafeh Rishon).
Writersoul: It should, though the Drasha doesn’t mention it explicitly. (Actually, the Drasha is only an Asmachta so it could be Chazal only instituted it for an older brother and not sister, though I’m not sure why one would be Mechalek.)