Sunder Charms, Resistance Break, External Resistance Modifier

Decrepify and

Amplify Damage both work at full effect vs sundered enemies.

Conviction and

Lower Resist work at 1/5 effect vs sundered enemies, as you noted.

Conviction works the same whether it's from a Paladin or through Infinity.

It would seem that way, yes. It's a bit inconsistent considering that

Decrepify and

Amplify Damage both work at 1/5 effect vs physical immunes (assuming you don't have a

Bone Break).

Maybe Blizzard really did think about giving physical damage characters a buff when they decided to do it this way (perhaps because there is no corresponding -X% to enemy physical resistance on equipment?), but I don't think we ever got any reasoning for this decision.

That's because they don't have any thought-through reasoning to give you.

The main reason for any changes nowadays is simply "omg, people keep bitching about everything so let's randomly change shit to other shit without thinking anything through", same reason

Cold Mastery has been added to the list of "only 20% post-sunder".

The latter is missing in ShadowHeart's explanation/listing for the sake of completion but the rest is dead on. Character/unit source doesn't matter in the slightest. The only thing that matters is what skill/gear/whatever is adding the -res.

That having been said, there is one thing to keep in mind as well when you compare merc vs. non-merc:
Your sunder doesn't work for your merc. So if you're looking at say a phys immune, you have a sunder and nothing else (decrep/amp) is applied yet, you will do your 5% post-sunder damage, your merc will do nothing. Once say decrep procs, it'll hit fully for you and your dmg will skyrocket. But to make that difference even larger, that same decrep proc does work for your merc at the normal reduced rate and since most phys immunes are easily broken, he will now also finally start doing at least some damage on top of your own massive boost.