Lockjaw is going to be pretty much all CGI if the set pictures are any indication.
Not sure how you would do Medusa's hair with practical effects. Her living hair powers would have to be CG.
Speaking not as a professional, the way I would first try it would be puppeteering small tentacles, perhaps 30-40 of them, an inch in diameter and 4-5 feet long, distributed throughout the wig-like prosthetic. Have it animated by a puppetering team in order to give the thing a constant feeling of life, drive the cost of just having her on screen way down and allowing for casual movements, touches, gestures and even picking things/people up. Medusa's CGI would be reserved for fighting scenes: whips of tentacles, slicing through things and a large scale area attack that occurs at one point in the series.
If that failed, I would do change her power slightly so to make her hair something more CGI-friendly, like, Rapunzel style, where it glows when she's using it.
I'd also do Lockjaw as a puppet/costume with CGI flourishes to add more life for close up shots. Triton would be done Abe Sapien style.
I really believe characters, as much as possible, should be characters and not SFX shots. Let them interact and be real, and I think that comes through. I think in a movie, where you have an extra 20 mil to burn and you can bring in Andy Serkis and his team, I think that's as good, because they're essentially doing digital puppeteering. I think CGI animation can be great for effects, shots, moves, events, but when it comes to characters, if you can make that thing real in the moment, then that lifelessness will come across and you'll have a weak character.