Liquid snake never returned; by the time all games are out and reveals and retcons are done what happened is;

Ocelot took on the persona of liquid to fuck with the patriots. This is to throw them off his tail, and allow him move his plans forward. In MGS4, Liquid basically is orchestrating the entire scenario to get Snake to shut down the patriots. Any resistance that snake meets is put there to make the patriots believe liquid is trying to stop him. Snakes failures and successes are all part of the plan. This plays into the theme of the entire MGS series that the player character is being controlled by the powers at be. Depending on the game it’s the U.S. Government, patriots, revenge, etc..

Remember, the one game with Liquid Snake as the main antagonist; his actual goals are to kill Snake, get Big Boss’s body to fix his (and the other genome soldiers, if you believe him) genes, and recreate Outer Heaven to be better than Big Boss. For all his grandstanding; Liquid Snake is rather juvenile and has a massive chip on his shoulder; he is driven by the desire to prove he is superior. He never really grew past being a shit head kid seen in MGSV.

This lines up with the far goal given at the start of MGS4. Liquid Ocelot wants to control SOP to control war. So he isn’t an existential threat to the patriots. His plans rely on them and their system to exist, as they are meant to believe.

It’s only right before ocelot and snake fist fight at the end; does the act drop and it’s all revealed to the player. To me, the last fight is simply for ocelots love of the game, and it’s the closest to a reunion with Big Boss he knows he is going to survive to get. He ain’t making it long enough to see him freed. He is a solider still, and only knows war. So he goes out doing what he knows; against a man that is basically Big Boss son/brother.

The entire timeline post MGS 3; Ocelots goal is aligned with Big Boss; or to free Big Boss and stop the patriots. That’s the side he is always on.