Yes and all of your hypothesis is subject to players skill and usage of two abilities. Also there are enough video of gameplay showing Puma is actually a counter for the Stuart and not the opposite, shell shock or not.
Now, the same way a 222 can catch off guard a M15 and kill it, a Stuart can do the same with a Puma which is good for the balance.
A 222 beating an AAHT is unlikely unless the AAHT player screws up hard. Stuart vs Puma is a much more even fight.
If the Stuart advances then the Puma reverses, popping smoke when Shell Shock is used. If the Puma advances then the Stuart advances back. At close range the smoke won't save the Puma because the Stuart can drive through it before Shell Shock wears off.
The Puma wins the chase. The Stuart wins the joust. The defender chooses which it is.
It's a much more interesting matchup than the RNGfest of AEC versus Puma.