What I'd like to know is: how much of my understanding about Heidegger is completely wrong, and actually just my own thoughts imprinted over his?
Oooh good question! This is something I should ask myself as well, for I have spent a long time forming an interpretation of Being and Time, which is interestingly quite different from yours, Arjuna,
I have embraced B&T as a work of ontological/hermeneutical importance, and tried to reconcile this with computational/biological theory. Your interpretation, through correspondence (which I have never heard of, but I imagine is philosophically close to hermeneutics), and centred on the essence of truth certainly seems to be a valid reflection of what I read in B&T, even if I don't fully agree.
As jgweed said, the language employed by Heidegger lends his work to flexible interpretation which has been, in part, the reason his work is considered both difficult and fertile. I do like your conclusion...
....To live, you have to have an ego. The ego requires firm footing in terms of defining itself. Its demands create acceptance of partial truth as Truth. It appears to be only the occasional fringe element that becomes unleashed from the demands of the ego long enough to sort out what's actually happening. It's almost a kind of de-evolution of mind. A suspicion I've lived with is that this de-evolution can happen at any time to anyone, but it may be that this de-evolution is actually a component of change. Backing up and out of who we are, allows us to then launch down another path. I suggest that the 20th century created a bumper crop of people like this.
Again, I am not taken to making such a psychological/sociological interpretation of B&T but this seems pretty decent.
Returning to your original/final question, this is something that falls in the realm of hermeneutic (interpretation) theory. If one believes in "truth", then there can be only one valid interpretation; think Christian literary definition of the truth with defining and changing the canonical gospels (which, however, still remain hermeneutically diverse). But a more rational approach is to accept that everything is relative, and thus interpretation is an act of dialogue between author and reader. Once a work is made public, there ceases to be any single "true" interpretation, but there will be as many as there are readers, each bringing personal memories and experiences to their own personal dialogue with what the author wrote, and each seeing what was written in a different light.
There is a lot more that can be said on this, but the upshot, as I see it, is that with no "truth", the notion of an idea being some kind of aeterna veritas, and having some "power" as a result, is demystified. An interpretation is only useful, good and worthy in as far as it is practically
useful. We should all question whether or not we have interpreted "correctly", for of course we do not wish to be "wrong", but in fact being "wrong" is tantamount to nothing more than having a different outlook on things; right or wrong does not alter the fact that thinking something has no effect on the world, and only through acting does our thought create "good".
I know that a lot of philosophy is idle, mental exercise and curiosity, which frequently has no need nor desire for action; but in this case, asking whether an interpretation is "correct or not", I think the above is relevant and important to appreciate.