Clicking ‘show info’ on an entity that is not your own ship will only ever show you the base stats with no modifiers applied (otherwise no one would ever use damage-type locked ships in PvP!).
Secondly, negative resist modifiers work by taking the given percentage of your current base resist away from you current resist. So a ship with 50% base resist would end up at 100-50-(50*0.7) = 15% EM resistance.
So the way this is phrased, 950 DPS is your ‘paper’ DPS (i.e. the amount that the game and/or Pyfa tells you you are dealing assuming 0 resists). This doesn’t take into account resistances, so against an opponent with 50% resistances you applied damage would be 950*(1-0.5) = 475DPS.
Do the same calculation with pure EM damage and 15% resistances and we get 770*(1-0.15) = 654.5 DPS applied to the enemies.
So going back to your numbers, the Therm/Kin ammo should give a completion time of 855,000 / 475 / 60 = 30 minutes (you dead!)
While the EM ammo should give you a completion time of 855,000 / 654.5 / 60 = 21.8 minutes (you dead!)
I suspect your HP estimate may be a little out as this also assumes no time spent ‘not shooting’ the hostiles.
It is also worth noting, that the only entities in Abyssal deadspace with perfectly even 50% resists are the drifter battleships and cruisers. Sleepers also have even resists, but not 50%. Everything else has a varied resistance distribution over the different damage types. Given a 70% resist drop, you are pretty much guaranteed that EM will deal more damage against all opponents, than any other damage type however.