How picky are mbunas if you miss a week? Let's say you go two weeks in between changes? I say this since my wife and I are avid travelers and sometimes are gone for two weeks. Generally only once a year.
They are fine for 2 weeks. Don't overstock beyond the level you need to manage aggression (20 for a 75G with mixed gender mbuna, #of species will vary depending on aggression of the species. You might have 4 species with peaceful mbuna like yellow labs and acei, or one species with aggressive species like kenyi) Just do extra water changes before and get your nitrates to zero.
So you will be zero when you leave and 10 at week two and finally 20ppm when you arrive home to change 50%.