firs off I would like to point out why the second set of runes consists of magic and armor pen.
poison dart = 50(first hit than 80 due to the poison effect from the first)
Malady = 20
ionic spark = 125
wit's end = 42
madreds bloodrazor = 4% max health(120 max to minions)
thornmail = 30% damage taken returned
ALL of this is magic damage ^^^
So lets see, for every 4 attacks we have

(50[+40% ability power{66 damage with rabidon's}]x4)+(30[+10% ability power
{16.5 damage with rabidon's}]x2)+(20x4)+(42x4)+(125x4)

=200(40%=10=40{264 damage with rabidon's})+90(+10%=2.5=5{33 damage with rabidon's})+80+168+500

=1083{1324 with rabidons}
+4% max health magic damage with this build, which would take you about 2 seconds(with this build)(not to mention another 30% of damage you are taking).

Than you can start adding your attack damage.
(782 with bloodthirster added on...= 1975.5 damage every 2 seconds[with no stacks]{626 with rabidon's = 2093.5 damage every 2 seconds}) and another 32.5{49.5 with rabidon's} per second for 4 = 130{198} damage.

A couple things to think about here would be, is that overall damage more important than the lifesteal sustain?? Also do you want your shrooms to do more damage? personally i feel that your shrooms do NOT need this extra power for a couple reasons
1. you lose that sustain from the lifesteal(from 38% to 18%)
2. your shrooms are not guaranteed to hit...where as your attacks are more guaranteed
3. oracle's Elixir....no more shrooms unless REALLY well placed(vision ward too)
And overall magic pen is 24 + 16.83(with runes)= 40.83%(max is 40% anyways) and armor 30%

If your game is going good build ionic spark instead of Executioner.

(at full stats)
Attack Damage 341
Ability Power 230
Attack Speed 1977
Movement Speed 470 + 26%/52% (move quick ability)
Crit Chance 75%
Crit Damage 852.5
Cooldown Reduction 15%
Armor 82
Magic resist 30