Yes, the monster's physical resist being sundered helps your skeletons too, confirmed as well as the -% physical res.
Without sunder, amp only is 1/5th it's effectiveness, but with sunder it works perfectly.
Ye, remember that I read it in Maxroll or somewhere. It looks like it too, as with the sunder and Amp dmg, monsters melt, even the physics immunies.
The protector stone however ( physical jewel). Does the minus physical res helps your minions?
If it does, the Amp dmg will even doubles the effect.