How does "elemental" damage on weapons work with "physical" skills?

Suppose I have a weapon that does 1000 (black) damage and 1000 arcane damage, and I use it with a skill that says "Physical" in the skill tooltip. And I have some gears that says "Physical skills deal 20% more damage".

From the animation of how monsters died, I noticed dead monsters have a glow of arcane color on them, suggesting they were killed by an arcane damaging source, even though all my skills say Physical on them. This made me concern about whether "Physical skills deal 20% more damage" is actually useful.

I then did an experiment with a friend in the PvP (brawler) mode. My friend equipped an amulet that prevents arcane damage and heals herself instead. I then used that said physical skill with my arcane weapon on her - and she took some damage and healed some, suggesting my "physical" skill had a physical component and an arcane component.

Does anyone know how exactly the damage is calculated? Specifically, is that "physical skills deal 20% more damage" working to its maximum?

Physical skills work a little bit differently than Elemental skills. Here is my current understanding of the difference:

Normally, using an elemental skill (e.g. arcane) will convert all of the damage from the skill to that element. It will then be modified by stats like "Arcane skills deal 20% more damage."

Here is a semi-official post regarding how this works:

• http://us.battle.net/d3/en/forum/topic/11986398637

Physical skills work a little differently. Some physical skills convert damage to the physical type, some do not, and some only convert some damage to physical. If you have a weapon with elemental damage (e.g. arcane), these skills may still allow the arcane damage to go through. The physical portion of the damage will be affected by the stat "Physical skills deal +XX% damage," and the elemental portion off the damage will be affected by elemental booster/immunity stats, like your friend's amulet.

I do not have an official source about physical skills, but here are some discussions:

• http://us.battle.net/d3/en/forum/topic/11914142180
• http://us.battle.net/d3/en/forum/topic/12427482723

Here is info on how to tell if a given physical spells convert damage type to physical:

Unless a skill explicitly states in its tooltip that it deals Physical damage it will inherit your weapon's element. For Elemental bonuses this means that some of the skills that are listed as Physical will actually receive an Elemental bonus that depends on which element your weapon carries.

For example, Multishot / Full Broadside is listed as a Physical skill, but it actually doesn't have an innate element. As a result when I use it with my Cold damage Windforce its damage is increased by Cold elemental bonuses.

Source: http://diablo.incgamers.com/forums/threads/request-for-elemental-and-skill-clarification.852649/, http://us.battle.net/d3/en/forum/topic/11986398679

I cannot find any official confirmation that this is true for all physical spells that do not literally say "physical damage" in the tooltip, but it's the most definitive answer I've found so far.