Characters Joining Harlequins

An Archon is attached to a unit of harlequins, which then moves through difficult terrain.
However, the Archon himself does not move through the terrain, only the harlequin models.

Does the unit still need to roll 2d6 (no move through cover on the archon when joined to the squad) for movement, even though the model that would slow the unit down is not going through terrain?

Well, at first look they should lose it due to "move with speed of slowest model" BUT all IC have free MTC/SR that is NOT listed in their special rules(page 47 of rulebook), so no, they don't lose it;)

What I meant was can the harlies still move the full 6", without having to roll at all.

Even if the ICs themselves don't enter terrain?
The way i've been using them so far is to have the harlies skirt along the edge of a forest, while the ICs move along beside them just outside the forest.
My stance is that as the harlies ignore difficult terrain, then effectively no model has gone into terrain at all. I can't find anything that explicitly stops this maneuver, though it seems like a bit of a grey area.

All models move at the slowest rate in the unit.
Even if you only move one model through terrain the entire unit is effected as if it had moved through terrain.

You'd end up moving 3d6 - the Archon's movement.
The rules don't allow movement effects to work with only part of the unit - it's all or nothing, and the Archon lacks the flip-belt.

Yep - 2d6 it is as harlies DO NOT have MtC... they ignore cover

Oh. Puck, I thought it's "normal" MTC. My bad, guys are right.