Feast is most-likely taken when you're laning against an opponent who has high harass-potential against you (for slightly more sustain). Although for all intents & purposes this becomes less of an issue once you know how to counter-pick & have experience laning against certain champions.
Expose-Weakness is meant more for a team-fight effect and I think it specifically has a description of your allies doing extra damages. I don't think it stacks (and if it does it probably requires more than one team-member hitting the target with the same mastery but not something that I can confirm at the moment as being conclusive [can probably be tested if you have multiple computers set-up, log in with multiple accounts, and check the numbers after running tests in a Custom-Mode match]).
Damages, from what I have read, are done after mitigation. Equal pen against equal defense, with 520 damage, adding the 5% from R3 Execution, and the 3% from Havoc, for a total of an extra 8%, becomes 41.6 extra damage, thus likely a total of 520 + 41.6 = 561.6 damage (not 600). Also note that Executioner only takes effect if the target is already below 50% health. With that in mind attacking a full-health target at 520 base-damage with equal penetration will only trigger an extra 3% between the two resulting in 520 + 3% (15,6) = 535,6 damage.
Double-Edged Sword should probably only be taken against a squishy composition or if your composition is tanky enough to be able to afford you time & mitigation to burst down your targets without getting hit too much yourself.
