My rough input is, I'd just do something like this, from the bottom (lowest level) up:
(key 1 = Khat's priority key 1, etc.)
Code:
Priority Key 
 |---> Step 1 (reset to step 1 after X seconds; adjust to your taste)
            |---> key 1
 |---> Step 2
            |---> key 2
 |---> Step 3
            |---> key 3
 |---> Step 4
            |---> key 4

Strike key (reset to step 1 after 5 seconds)
 |---> Step 1 (do not progress to step 2 for at least 2 seconds)
            |---> strike 1
 |---> Step 2 (do not progress to step 3 for at least 1 seconds)
            |---> strike 2
 |---> Step 3 (do not progress to step 1 for at least 1 seconds)
            |---> strike 3

DPS key (pressed OR released) 
 |---> Step 1 ("reset to step 1 after 2 seconds" just so your normal first "hit" on any new encounter will be the priority ability(ies).)
            |---> Priority key
 |---> Step 2
            |---> Strike key
Considering the vagaries of ability timings, that will give you a close approximation of "random" without getting all insane with complexity, meaning it's manageable. And it "weights" the strikes toward Festering Strike. You can adjust the progress delays to tune it so it works better with your spam rate and results.

My 2 copper after a quick think, anyway.