Prone condition in 5e says the only move you can make is crawl, or stand up thus ending the condition.
Thorn got his Action with the run up and throw, so now we are in Fingol's time (in turns). He is giving it everything he has to get away, including Action Surge:
1) Action 1: Try to Blind (failed that)
2) Action 2: Try to push over and get just a bit of time to move (successful)
3) Move and Bonus Action: Haul ass to the boat, jump in and give a big enough heave to get away far enough into the water for real separation. Using the Cunning Action to take a Dash as a Bonus Action.
So I dig your series of Actions, sounds good and we should use them if you want to. It's just that we need to adjust that Thorn has to take some time to get off of his back after getting shoved over. It's not half movement, it's a loss of Movement pretty much as he gets back on his feet.
But then he could spring into action, make an aggressive movement, and then deliver all of the attack series that you laid out above.
I say, for theatrics and for an effort to be fair on the rules (because really Fingol should get further away with the Move and Dash while Thorn is just getting up), Thorn does it all if he can and wants to do one more thing:
Huge Jump, off the pier, crash into the boat with a mid-air Feint attack coming straight at Witchboy's belly. He's hurt and bleeding, it could be the endgame.
That, or run out to the pier, and do something else (stop, throw, etc.)
A big jump I think could be STR or DEX. You pick.
Thorn assesses the difficulty. It's heroic, athletic, but not impossible. DC 13.
And like Ez said in Admin, you guys are actively pursuing a Motivating Contract.
Your call.