Yes, but I used Lightning, because it does an even split between physical and lightning (so does fire and chaos), but Quelaag's, as an example, does more fire than physical, and would as that be better than a Fire/Chaos/Lightning, because you get more from having one do more damage than both do equal.
These are the sub-categories of physical resistance, but all physical damage only does ONE type of physical damage. Swords do slash, clubs do strike, rapiers etc do pierce. There's no multiple check there.
That's why I went through the math to show you how it works when resistances are exactly the same, and then to go on to show how it might normally be. Both show generally when the AR is 20% lower than the combined if a two element hitting weapon (250/-/-/250 means you need 400 in physical to out-damage it against most opponents
This is why the math is important. The reason why it seems like this is because the higher one damage type is (and up to 10 humanity increases fire damage, and nothing else), the less resistance will make a difference.
No
Why would you go +15 when the enemy has high physical resistance?
If you'd read my calculations in my last post, you'd seen that it's actually the complete opposite. The less resistance an enemy has, the more damage a two-way split weapon will do. Of course split might be an overall easier way of getting through the game, but it is rarely the optimal.
Again, see above
If you check the link to my huge post in my previous post, it's a post where I do calculations on how good resistance rings are, and you'll see that resistance does very little in negating damage in normal play. If you put on the ring, you'll likely last 0.3 hits more, so it won't really be a life saver. Run with Favor and Protection ring and Mask of a mother for highest survivability, as you will survive way more hits with even only the favor and protection ring than any resistance ring will ever do. They're generally a waste of a ring slot. Even if you wear Havel's Ring instead you can easily get way more resistance and still be light than you could with a resistance ring.