Eastmarch makes sense; it's next to Morrowind. I'm not sure which hold it would have been vassal to, but it seems to have surpassed it. So which hold was Hjaalmarch vassal to and who and where were the people on the other side? Reachmen? Bretons? Aldmer? Falmer? Dwemer? Nords must have started recolonising Skyrim from somewhere along the northen coast which leaves Haafingar, Hjaalmarch, the Pale and Winterhold. For that matter, are the nine holds of equal political standing, in theory? I don't really know how marches work so do correct that vassal idea if it's wrong.

A march generally means that it is a frontier between two rival nations which in this case was probably the frontier between the Nords and the Direnni Hegemony. Therefore the names Hjaalmarch and Eastmarch probably just stuck after time.

On the political standings of the holds I think Haafingar and Eastmarch hold the most power over the rest.
I.e those loyal to the Empire are basically sub-holds to Haafingar whilst the Stormcloak aligned holds are tied to Eastmarch, whilst Whiterun stands between the both.

The institution of kingsmoot Gives each jarl (and let's remember that we're loyal to jarls, not holds) a voice in determining the high king. Not sure if it's a vote ( it seems likely)-if it is, jarls might all have equal power.

March may be a corruption of marsh.

Perhaps because the area is a marching distance around the hjaal river.

is it spelled marCh ingame?

id assume its just a misspelled marSh, since its boggy and all that

I hadn't thought about it being a corruption of "marsh", but then that is germanic word, so that makes sense. And if the Nords started their reconquest of Skyrim from Winterhold, there could have been a march between the Pale (wasn't that a similar type of region in Ireland anyway?) and the then north-easternmost corner of the Direnni Hegemony which would become Solitude.