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So, I was playing a game against Tyranids and an interesting situation came up:
I assaulted a Hive Tyrant with some Biker Nobs and a Biker Boss. All the Nobs save 1 were at the left side of the Boss. The Hive Tyrant issued a challenge, and I accepted.
On his turn, he assaulted with some Gaunts on the right side of my Boss. The Nob on the right died.
My turn again, still locked in combat. His Gaunts are in base to base with nobody they can hurt, seeing as my Warboss is in a challenge. On their initiative step, they would be able to consolidate to base-to-base with my Biker Nobz. Does his Hive Tyrant get rerolls because they are currently not in base to base/engaged with a valid enemy?

They can't consolidate, they're still locked in combat. They can lend Moral Support though (see page 65).

Pile in, my bad. The 3" move you take to get into base-to-base.

The IC isn't considered another unit in combat any more.

Regardless of whom the non-character models are in B2B with they may attack, since they are in B2B with the unit. The Challenge only restricts who the wounds may be allocated to, not who can fight overall. Only if a character was without a unit would the non-character models be unable to attack and use Moral Support.

The termagants and the nobz will fight while the challenge goes on. Once the gants made BTB with the biker, the entire unit was now in combat and would have to pile in to each other as required.