Warrior's bash, slam, and kick-stun differences
 

At 60 as a warrior, is there any difference in the effectiveness that bash, slam, and kick-stun have at stunning? Do they have different chances of stunning? Different miss rates? Different duration? Different behavior when attacking from back? Differences in pvp vs pve? etc

Same everything.

Slam for large races lets you bash without a shield but after kick starts to stun there shouldn't be a difference in anything other than potential damage based on your skill.

Kick skill caps at 210.
Slam/Bash skill caps at 240.

At 55 a warrior's kicks gain the ability to stun, but with slam/bash having a higher skill cap it is the better option.

A non slam warrior should be swapping in a shield to do a bash to interrupt caster spell casts. Swapping weapons is a huge part of playing a warrior. Set up your hot bar to have your primary, secondary slots and a bag you can open by it.

Do the different skill levels mean a different chance for the attack to miss? Or is there something else affected by skill level?

Higher skill level gives you higher chance to pull off the attack. Thus, higher the cap, inherently higher effectiveness.

While Naethyn is right, I stopped quickswapping to shield bash when I hit 55. Call me lazy but ehhhhh.