if we're entertaining the theory that Hayate's daddy is Isamu, there're perfectly legit reasons why he's an Immelman & not a Dyson.
a - maybe his parents were never married & he grew up in a single-parent household with his mom, thus having the mom's family name; at the same time, dad's obviously absent but kept in touch from time to time. Isamu being the playa, this isn't an unthinkable possibility... sowing wild oats & all
b - maybe his parents divorced & his mom remarried & Hayate adopted his stepdad's family name
i've a feeling this mystery, like Lady M's identity, is gonna crush under the weight of my unreasonable expectation.