I think your action economy would work just fine. None of that sounded too outrageous to me.
Your read on how halving TNs work for traveling out is correct, but don't forget that Mythos Events "reset" the waters. The goal is to drive the players further out so they encounter more and more horrifying events.... Then do it again!
I don't think there is a big issue if you want to create a free one point Fisherman Profession that everyone is given for free. Especially with my recent update, where I reduced the number of starting Profession points. I'd be careful with allowing players to improve their traits, though; the game is intentionally designed so that the only "improvements" come from losing Sanity and gaining Dark Traits. You don't want to take away that carrot too much.
And you're spot on, if you only have a single Profession trait and it's "Dentist"... you're not going to be very good at fishing. You're reliant on your Personality, Drive, and Tools at that point. The Risk vs Fish mechanic is intended to be a lot more mechanical than narrative. It's just an excuse to get the players out to sea to experience spooky stuff. I didn't want to encourage the game to dwell too much on it.
Creating price lists for more things at Port is a fantastic idea. It's something I wanted to include but ran out of room. I gave myself a strict 40-page limit so the book could be easily saddle-stiched if printed. I print copies at home and bind them for my friends and playtesters, and going over 40 pages makes the booklets bind poorly.
I'm thrilled you're excited about it. When I wrote this, I assumed it was too niche to appeal to many people. It was just me making something I wanted. haha