My personal thought is
1) teaching beginners and intermediates is much easier on skis that are narrower than 90mm waist
2) improving your own skiing (fundamentals work on groomers) is MUCH MUCH easier on skis that are narrower than 90mm waist
The ski I'd pick for fun-skiing (100% fun hog) at Whistler is totally different from what I'd pick for instruction and personal skiing refinement.
Why would you doubt the person who is directing the instructor course?
The 3 skis you mention are totally different from one another. The Mantra is demanding and would be a lousy teaching ski unless your own personal skiing is pretty damned bombproof. The Rocker2 90 and Crusade are closer in character. Something playful at slower speeds is more fun to teach on unless you're going to teach upper-level skiers. If you're just learning how to teach, you're probably not going to be teaching many (if any) upper-level skiers at first.
Stab in the dark: maybe consider the Rossi Experience 88? Seems to me a pretty good mix of ease while teaching newbies, while having performance reserve for fun skiing.
(No, I'm not a Rossi pro.
At an alpine clinic I attended a few weeks back, PSIA Alpine Team members Robin Barnes and Eric Lipton were on Head Rev skis, I think Robin's were Rev 85 and Eric's were Rev 75. Either of them could ski anything. Why did they choose such narrow skis?