Go for it. I don't believe your cohort to be OP. Though, given your cohort is from Leadership, I note the same caveat as before: support/RP role unless special circumstances (e.g. I force them into an encounter).
It's worth noting the wording of the whip would, when combined with extending, include all ranges under its maximum range. "In addition, unlike most other weapons with reach, you can use it against foes anywhere within your reach (including adjacent foes)."
My interpretation is that the given range is 10 ft. but the spiked chain has the special ability to strike at adjacent foes. So extending just pushes the range up to 15 ft. and does nothing to disagree with or override the special clause.
Spend the XP like a buy off. Per the wording, you explicitly don't have this count as a level.
All MIC material is automatic yes.
Added.
It occurs to me the wording of abjuration's general rules specify that, from the perspective of the prismatic wall, it can't be moving forward to touch someone. So thus ramming is precluded—it would just cause the walls to wink out?
I'm not sure I can accept this ruling. A prismatic sphere flying through a swarm of flies should vaporize them, not lose the shove-fight.
Added.