Just hit 21 and still stuck at 85. Have not seen a bash skill up since I last maxed it at lvl 16, right before the patch.
I haven't seen Bamzal's problem yet, but yes, bash/slam should always interrupt if it lands:
"Slam:
A version of the bash skill. Shaman don't increase skill in bash, so a lot of people disregard it. They shouldn't - since Shaman get no type of stun or knockback spell, this is the only way besides weapon melee that we are able to interrupt spells. If you land a slam, it interrupts your target’s spell 100% of the time in both PvE and PvP. Without slam, most of those pesky halfee druids would have gated away on me."
http://www.notacult.com/guide_trollshaman.html
Stun immune should have no effect on it. Ogres won't get stunned, but the slam still interrupts spell casting.