4. Here's a few shots. the second is enlarged, brightened, and sharpened a bit.
5. In looking back at the dvd, Magnum sees the reflection of the hotel where he is, not Melody. (Mauna Luan
condo building in real life, located here
in Hawaii Kai). That tells him that the kidnappers are right across the street. The real life house here
isn't exactly "right across the street," but certainly close enough for the show's purposes considering they really could have shot it any numbers of places far away from the "hotel" and made it work. I've been impressed more often than not that they presented locations in the show much as they are in real life.
And right after TC says "We're on our way" I assume time passes before Big Jim cuts off the car, TC, Rick and Higgins arrive, and Magnum goes to the house and gets Melody. The show kind of indicates this with the music and panning shot of the front of the hotel over to the street, just after Magnum leaves the hotel room. Maybe they didn't have time left in the episode to show Magnum getting to the house and Big Jim and the other guys taking time to arrive. That would have made it clearer, but definitely there was five or ten minutes needed in real life that they didn't show.
7. I looked briefly for the house were Tina was staying but didn't find it at that time. I'll be going back to look again.
The other female was Tina's cousin, Evelyn Perry.
9. I don't know about the beach, to me it looks about the same as it did in many shows. And remember the lenses used on the cameras can make things look slightly different, I don't know if that played a part here. The stairs were indeed pretty covered with sand, and it looked like sand was all the way up the smaller sea wall. I think it wasn't always like that?
I'll let others respond to your other queries. My field of expertise involves things which I can find easily on the dvds.