It appears that you and I agree.
If after a hit in that end game, challenging content the character has 2 hit points remaining rather than none, is that important? If I am playing the character and he now has the chance to heal -- when being at 0 he would be unconscious and unable to do so -- I would consider DR to have been meaningful.
I agree that as a component in overall defense DR is the least significant. That does not mean it is not meaningful.
The cell formula is already there to incorporate DR so why not use it?
If monster attacks are at the rate of ~20 per minute and the probability of a hit is 40% then we expect 8 attacks to hit and 12 to miss. The fraction 12/8 tells how many misses occur for every successful hit. The value 1.5 is used as a multiplier to the speed of the monster's attack to tell us how much time we have to take an action -- in this case 4.5 seconds.
In game the monster is not going to engage in half attacks. It means that damage is coming on average in pulses of 3 seconds and 6 second. This sets a minimum time for (self) heals at 3 seconds.
If mitigation is not sufficient to reduce the incoming damage to less than the character's hit points all of this information is moot. The character has to be alive in order for the 3 seconds of time to have any value.
Next, the amount of healing must be enough to lift the character's hit points to more than the next amount of damage. If it does not then the character dies on the next hit regardless.
The model cannot assume that every form of (self) healing occurs simultaneously. Players must take action sequentially. Neither can it assume that sequential actions take place without delay -- players still must press keys and the electrons must still communicate to the servers.
I do not think that it is probable that any character is capable of (self) healing in end game, challenging content. I think that will be obvious when actual damage numbers from monsters are applied. I think at best (self) healing will be practical for some characters against the preliminary monsters -- the trash mobs.
However, none of the melee builds used as samples will have sufficient healing to recover in a boss fight. In fact at 3000 damage (which was identified as low end LE trash damage) the two rogue builds die immediately upon being hit.
All of this suggests that trying to incorporate (self) healing in the analysis is not a productive next step. It suggests that the only viable healing method is external healing from players running characters designated for that role.
And, if that is the situation then it is better to evaluate DPS than (self) healing since the melee roles in legendary content are those of glass cannons.