And yes, it would kind of make him immune to dying... in cases where he could be accidentally rescued.

By a time traveler, who doesn't need to do much - only to appear and make the paradox possible. That makes the list of possible cases much more extensive than a first year student locking the door.

And here's the line I was referring to:

Correct me if I don't remember something, but that episode didn't imply that he would be able to create and manipulate tentacles like he did in the book's final.

And that partial transfiguration thingy is a DEM itself as it was used several times to get Harry out of hot water and serves no other purpose in the plot.

The end of the book looks like Harry's worst case of self-assurance and indiscretion to me.

it's entirely possible that if Harry hadn't gotten out to pass a note, someone would have gone back in time to investigate his death, and inadvertently caused a paradox by unlocking the door.

Doesn't this make Harry virtually immortal unless something so catastrophic happens that it destroys all the world at once?

in chapter 28 when he used transfiguration to apply force.

In this case the goal wasn't achieved, because the ending of the book was Harry's worst failure.

For instance, when Draco Malfoy decided to torture Harry to death and the only thing that saved him was time machine and time paradox. Very literal deus ex machina.

Another one is the end of the book in all its entirety. Without a very helping hand of the demiurg, Harry should have died (and teach us one very important lesson).