I'm hoping to try and answer the rest of the questions later today or until someone else can chime in:
1. When does the split occur? Is it:
- Half of the base damage from enemy player with individual mitigation?
- Half of the mitigated target damage with additional tank mitigation?
- Half of the mitigated target damage without additional tank mitigation?
These are the stats I used for my calculations.
Tank damage reduction: 35.31%
Tank Defense Chance: 24.07%
Tank Shield Chance: 36.62%
Tank Shield Absorption: 45.02%
Targeted player damage reduction: 26.95%
Enemy player Surge: 67.42%
All players: 551 Expertise
I will be using Hidden Strike for this test. Hidden Strike is one-tick Tech damage and cannot be mitigated by D/A/S stats even without guard.
Normal unguarded attacks to targeted player from Hidden Strike:
1428 average normal damage or 1812 raw damage
2390 average critical damage or 3034 raw critical damage
Non-crit Hidden Strike:
Tank: 638 / 863 raw
TP: 681 / 864 raw
Raw damage after 5%: 1727
Raw damage before 5%: 1813
After about 15 non-crit Hidden Strikes, the tank consistently takes less damage than the targeted player due to its increased damage mitigation from armor.
Crit Hidden Strike on targeted player:
Tank: 630 / 852 raw
TP: 1134 / 1439 raw after surge
Raw damage after 5%: 2308
Raw damage before 5%: 2423
Critical and surge rating are calculated after transfer. Great news for tanks: Only half of the damage from an enemy attack will count towards a critical.
Crit Hidden Strike on tank:
Tank: 1069 / 1446 raw after surge
TP: 678 / 860 raw
Raw damage after 5%: 2306
Raw damage before 5%: 2421
Tanks can also be victim of a critical even though the targeted player will not be. Does that mean that the attacker has twice the chance for a critical? Technically, yes, but remember that it's calculating Surge from the split damage. The chances of a dual critical will be half of the enemy's critical chance just to deal the same damage unguarded.
With these numbers, I can conclude that the enemy attacker will first have its damage reduced by 5%, then split in half as two attacks between the two players. Each player uses their individual damage mitigation resources (e.g. armor, cooldowns) to absorb the resulting split damage. Each attack has its own chance to critical but will only calculate its own split damage.
2. Are D/A/S stats factored through Guard damage?
For this portion, I will be using Overload Shot. Overload Shot is one-tick energy damage and is mitigated by D/A/S by normal means.
Normal unguarded attacks to targeted player from Overload Shot:
831 average normal damage or 1054 raw damage
1350 average critical damage or 1713 raw critical damage
Non-crit Overload Shot
Tank: 344 / 465 raw
TP: 380 / 482 raw
Raw damage after 5%: 947
Raw damage before 5%: 994
As stated in the previous question, tanks consistently take less damage than their targeted player because of their armor.
Possibly Shielded Overload Shot 1
Tank: 190 / 279 after absorption, 377 raw
TP: 407 / 516 raw
Raw damage after 5%: 893
Raw damage before 5%: 937
Possibly Shielded Overload Shot 2
Tank: 193 / 280 after absorption, 378 raw
TP: 368 / 467 raw
Raw damage after 5%: 845
Raw damage before 5%: 887
Possibly Shielded Overload Shot 3
Tank: 196 / 284 after absorption, 384 raw
TP: 410 / 520 raw
Raw damage after 5%: 904
Raw damage before 5%: 949
The last three shots indicate that some form of shield/absorb chance may be in effect. We can see that the targeted player takes normal damage but the damage to the tank is almost halved. This would lead me to believe that shield and absorb does work, counter to my previous statement, but it will not indicate it as being successfully shielded or absorbed anywhere in the game. This is proven as shielding Guard damage does not reduce charges of Dark Ward.
I can also confirm that Defense chance will be calculated through Guard damage so long as the originating damage could've been mitigated (i.e. Melee/Ranged but not Force/Tech). You can easily prove this by dealing damage to the targeted player and see that, occasionally, damage towards the tank will not be applied at all. One caveat: This will not show up on a parser, and you will not see text indicating that you have dodged a guarded attack.