I would definitely agree the final scene is too truncated. They should have made the link to MG1 far more overt. It feels like it's trying really hard to be confusing/obtuse in the final scene before VS walks into the shadows. I think it's reasonable to go by what the game implies, though. The series has implied plenty of important plot points in the past. (... Or maybe it hasn't. And that was always part of the problem. Good stories do it, anyway. It allows more freedom for meaning and discussion.)
I think the Liquid and Ocelot things can be 'explained away':
- Liquid does spend time in the Middle East. Literally one day/evening during the OKBZero mission (lol). Srs, there are so many things mentioned in the games, he probably spends time there elsewhere. Where is the Episode 51 island located actually?
- I think the Ocelot angle is covered fine in an MGSV tape he says torture is all about precise, directed use of threats or pain to get the information you want (how he approaches Huey). He tells Kaz to stop when Kaz is getting bloodthirsty. That's not what it's about for him. But when we see him next (in canon) in MGS1, he is not being himself. From MGS1 he is being a spy, in-character acting as a bloodthirsty intel operative. He "loves torture" from MGS1 onwards because he is playing a violent, vitriolic character. A villain. (Not because he really does enjoy it.)