If I'm crusing along at 100 KIAS in a 172 in clear air and I roll left
while maintaining neutral rudder, what will happen if I don't
neutralize the ailerons?

I'm picturing the plane rolling on to its back while losing altitude
and either completing the roll (doubtful) or stalling into a dive and
recovering in the other direction , right side up. (BTW, that's a
split-S, isn't it?)

I wouldn't anticipate a spin unless some serious rudder was cranked in
the direction of the roll.

-Scott "I feel like REALLY flying today" Lowrey