challenges from characters on different base sizes

okay, here's an example:  A unit charges a horde with skrag on one side.  They contact so that they are not touching skrag.  There are no other characters in Skrag's unit.  Skrag issues a challenge, but cannot make-way to be touching the unit.  what happens?  Are they free to ignore the challenge, or must the unit slide over to make contact with skrag?

Slash can hop to make way to the.other side of the unit tobe in contact

The model who is accepting the challenge has to move over to the challenger.

Elves example:
This is perfect gameplay, when a lone character is charging your horde unit with lvl4 mage. He makes sure he is charging in, to make B2B to the Lvl  4 character. To take him out in CC.

He is charging and has first choice to challenge or not, but he will not do so as he then cannot kill the lvl4 in his turn... being afraid for the magic phase in the following turn.

But now your captain is calling for a challenge and the lone character has to accept and has to move over to the other corner where the captain is.

So to the quetion of the OP.... If Skrag is challenging, the model who accepts, has to get into B2B with Skrag.

perfect example, thx