This is a very literal question that has puzzled me since I got the book, and now it looks like I get to run Barrowmaze on short notice.

From the regional map (Barrowmaze Complete pg 6), the village of Helix is north-northeast of the mounds.
From the overground map (BC pg 29), the the hex marked "start" and all of the lower-level-compatible mounds are in the southwest.
They're travelling slowly through the swamps and don't really have time to circle the moor.

How does that even work?

The text is consistent enough I assumed the maps weren't unlabeled, but ...

I've been running Barrowmaze for years, my players might even finish it this year. I'll be a little vague on the beginning, but the Barrows are in the middle of a swamp, the players will take a meandering path to get there.

For the first few trips, I had the players hire a guide (son of a local ranger). Have the guide take them to the "usual" main entrance, point a few isolated barrows which have been broken into (and mention that with digging equipment, other barrows could be dug into).

The worst part about the maps is they do a crap job of highlighting alternate entrances into the barrows; I had to go page-by-page and draw on the maps the barrow numbers the exits go to.

I've used Barrowmaze I as a starting dungeon for years.  Not having Barrowmaze Complete, I can't really speak to your map questions other than say this: start them at the edge of the Barrow field (at which point is up to you) and let them immediately get to exploring.  Use flashbacks to describe meeting their guide, gathering rumors, tracing their route from town, etc.

I think the whole idea with the geography of the barrowmounds is that when parties leave from helix they have to pass the mounds (from the east) to get to the start location (west of the mounds).

This is because the moor is quite boggy (the mounds are only accessible in the dry seasons) and the mounds are surrounded by deeper waters. The original entrance to the mounds was from the west. This is likely due to the ancient civilization having been to the west which is why the path meanders from that direction into the mound area. (I might have added a path that lead to the barrowmounds from the west, specifically to mound 12, and from a ruined site in the blackened forest to the west, which i made another adventure location)

It's actually a good thing, because you can describe having the party have to pass the mounds, adjacent to them. They get a sneak peak as to the scope of the mound area and you can set the atmosphere... Grassy islands pockmarked with stone protrusions within a sea of boggy water... A lone, hunched troll walks between two mounds in the distance... There's a particular mound the party may pass that contains trolls on the southern side. There are necromancers on the northern side, as well as the hidden dragon mound.

A party with the means can also swim\wade across if they feel emboldened to, but I would not advise looting those mounds at low level (but then they'll learn their lesson).

I believe this was recently discussed on the Barrowmaze Facebook group, where Barrowmaze creator Greg Gillespie chimed in.  

https://www.facebook.com/groups/434718316655624/

See the post on September 26