With the different endings I've heard it's possible to beat the game without knowing you are in the final battle. Is there a point of no return that's made obvious?

It should be obvious to you after beating a boss and returning to the hunters dream. How anyone could just go into the final area and not realize that was happening is beyond me honestly. Its the first time something in the hunters dream changes, and the Doll tells you to go talk to someone in a new area.

I would amend what @sterling said to, 'It is the first time something in the hunter's dream changes IMMENSELY,' as there are a handful of tinier changes that can happen through the course of the game - all of which easily pale in comparison to the Big Wumba at the end.

Since his name has been mentioned in many threads now, I don't consider it a spoiler, but just to be safe: don't talk to Gehrman in the flower garden until you are ready.

Not really, but you can accidentally skip the final boss.

I knew i was pretty much at the end but thought I had one more boss to go so I accidentally beat the game. Which was a bit of a piss-off because there were sections I knew still had locked doors I hadn't gone through (including upper cathedral ward, which I learned about after). Oh well, just another reason to go through NG+ now.

I am fairly certain I've reached the point of no return since MY HOUSE IS ON FIRE. As long as I don't initiate the talk at the great tree I'm free to do as I please correct?

Correct.

Yeah, you can explore at your leisure so long as you don't begin the talk.