+% life vs +% resistance

I am afraid to say the EHP formula you quoted is very wrong. EHP should be HP divided by combined damage taken rate, not multiplied by damage reduction rate. For example if you have 10k HP and 60% combined DR, which means you're only taking 40% damage, it is equivalent to having 25k HP without any damage reduction, so your EHP is 10k / (1 - 60%) = 25k.

Using your simplified model and numbers, base EHP is 50k / ((1 - 56%)(1 - 78%)) = 516529.

With Invigorate, total EHP is 1.1 * 50k / ((1 - 56%)(1 - 78%)）= 568182, or a 10% increase.

With Impunity, total EHP is 50k / ((1 - 65%)(1 - 78%)) = 649351, or a 26% increase.

As @yx pointed out, every 300 resistance equals to 100% EHP*, so the only case Invigorate will beat Impunity in terms of EHP increase is when 50% of your resistance is less than 30. Invigorate unlocks at lv41 and usually a character don't have much resistance (or HP) at that time, and the 310 regen could be something. However when a character reaches lv60 and enters Inferno, Impunity is the clear winner.

*Based on assumption of mLvl=60, while your DR numbers were calculated based on mLvl=63, so there might be a small gap but doesn't change the big picture.