1) So Duskbalde could work, but you want the most damage you can possibly have. If you are fed enough to oneshot people without the damage bonus from Prowler's, you can go ahead and buy Duskblade. You won't be able to do insane combos w/ your Q or E, you might just be a better overall assassin. Eclipse just doesn't fit him. It's more of a duelist item than anything else. You won't benefit from the lifesteal and shield, so it's just not a great buy.
2) I mean the cooldown reduction is probably just a better rune. I personally don't take it bcs it's just so funny seeing Ornnrun around the map like if he was Kayn. Also, Ornn has low cooldowns by himself, meaning you might not need the extra.
3) It surprised me too when I discovered tis build, but you actually deal the most damage w/ your basic attacks. AND you most likely won't be fed enough to oneshot people anyway, so I just find the attack speed better.
4) So this one is harder to answer. I choose electrocute bcs of the early lane damage. Both Dark harvest and Predator could work under certain circumstances. If you wanted to go Dark harvest, you should go jungle or bot. On top or mid it wouldn't work, bcs this build relies on people thinking it sucks. In jg or as the ADC they will think you are trolling, therefor the won't be scared of you, letting you get a lot of stacks, which could benefit you in late game more than Electrocute (not sure about it though). With Predator you should go either jg or mid. If you go mid, play it like you are the second jungler adn leave lane at every opportunity. Predator is a risky pick, bcs it really depends on how many kills you get bcs of it. If it's like 5+, it's worth it. Otherwise you wont be able to do anything in mid-to-late game. If you trust yourself, then go for it. Otherwise I would recomend starting out with Electrocute and later experimenting with other runes.

Did I answer everything? I hope so. If you have any followup questions then don't be shy, just ask. I don't bite.