Bond dives underwater to meet Melina and update her on what he's learned about Cristatos. In returning to her ship, she removes her SCUBA tank and leaves it at the bottom of the ocean. Later in the film, it's available to save their lives. WHY WOULD SHE LEAVE IT THERE IN THE FIRST PLACE?
She's Genre Savvy enough to know it might be necessary?
To be honest, this seemed obvious to me from the first time I saw it: It's a place marker. Underwater Ruins are just like any other archaeological site - they must be examined inch by inch. Ultimately, Melina is an archaeologist, and those ruins are huge. She's close enough to her boat that she has plenty of air, and in shallow enough water that she won't get the bends. So she's leaving something large enough both to be seen and that it won't drift away/be eaten by sharks or other fish by the time she comes back to pick up where she left off.
Assuming that it is Blofeld (he's unnamed, presumably due to the copyright issues surrounding Thunderball), when does the teaser for this film fall?
1981. A deleted bit of dialogue (likely for Lawyer-Friendliness) has Blofeld explaining "I've been waiting 10 years for this day." What happened 10 years prior? "Diamonds Are Forever."
Because she's Bond's wife, he's from London, and as her next of kin, he would have the rights to dictate where she would be buried.
Also, she didn't live in Portugal- she and her father happened to be there when they met Bond. In fact I think she was staying at a hotel. Her father is an international crime lord- its not a stretch that both of them lived in several places, so Draco probably wouldn't have a problem with her being buried there. The fact that Tracy is played by an English actress and speaks with an English accent might be enough to Hand Wave an explanation that she grew up in England, or spent a lot of time there, even if she isn't English by birth. More reason to bury her there.
Why didn't Bond destroy the ATAC while it was still in the wreck? His mission wasn't to recover the ATAC, it was to make sure nobody else did.
It's pretty hard to see how he could have done so in a way which a) made sure that it was destroyed and b) didn't risk his own and Melina's lives?
Presumably he just never considered it and figured that he could simply grab the ATAC and run. His pause when Gogol arrived seems to indicate that he was unsure about whether or not to destroy the technology.
This, but added to the fact that there was no reason to destroy it in the wreck. High-tech equipment costs money to build. At that point, he didn't know about the JIM diver coming for him, and once he got away from the JIM diver, he didn't have a chance. Not until Gogol arrived with a gunman did he have a choice between turning over the ATAC or getting killed or perhaps to Take a Third Option, which is what he did.