Protector's Stone in helm still worth?

Even though the ED will be bugged, and other stats aside, is it still worth doing this for the minus enemy physical resistance? Specifically for ES/MB Warlock and/or WW Barbarian.

As a blanket statement:
Absolutely.

-10 phys res is quite a bit. Plus the other non-ED stats still work.
And depending on your helmet, you can still make the ED work as well.

Whether it really is worth it for your exact build depends on the build as it can be counterproductive if say you lose a breakpoint that ends up netting you more and that you can't get by other means. Especially for an ES lock in the current state of the game as you can rely on nearly 100% amp uptime, making -res less impactful.
More likely than not though, at least with some shuffling around of other gear, it'll still be worth it.

That said, if you can slot it into your weapon directly, that'd usually be the better way to go.

WW bar, you need to see your monster fight status before use it.

Usually it work good for ww bar when equipment is not so good.

if you see 3%-5% hp remaining in your monster fight. it may help. most case, no effect.

The other scenario is against sunder phy monster. it will help a little bit but you still need spend some time to kill sunder phy immunity.

Also, please experiment it with worst phy Stone if you feel perfect one may help. This jewel is not strong enough. most case, worst jewel = perfect jewel.