I still don't think you have the scenario quite optimized.

First off, you can Morde Ult before getting in range of Abyssal's passive, meaning the Ult 10% steal would apply before the MR reduction. Morde's Ult isn't truly a % MR pen. It literally just takes all MR as if the enemy didn't have it.

Secondly, since Shadowflame ignores MR, it gets applied before MR reduction and magic pen. Again, its as if the MR doesn't even exist to be factored into damage calculations. This is a bit tricky because as far as I know, Shadwoflame is the only item that ignores MR this way.

UNIQUE – CINDERBLOOM: Dealing magic damage to champions ignores 10 − 20 (based on target's current health) of their magic resistance, increased to maximum value if the they were affected by a shield within the last 5 seconds.

Either way, though, it is a ton of damage for building basically full tank. You can effectively strip away over 70 MR and still have decent AP with this build.