Great job!
I would suggest practicing with ALL the dumps on your BCD until you get familiar with locating the pulls and their location is second nature. It can be a little tricky at first finding a particular dump if you are not familiar with them. Usually you'll find some kind of knob or ball on the string. However, I have often borrowed gear from tech divers which have cut off the "knob" from the butt dumps on their wings because it is a snag hazard. The first time I did that I was trying like crazy to find my dump and I thought that it was snagged in my gear and "buried" because it was hard to feel just the thin little string through thick gloves.
However, after awhile (with practice) I could find it easily.
I also have split my weight up since I dive with a well-insulated drysuit and lots of lead. That is a great idea.
A good opportunity to "tweak" your weight is at the end of that dive since your tank was pretty empty!
On an "empty" tank, holding your breath (full lungs), and no air in your BCD you should float vertically with the water crossing the lenses of your mask in the middle. If you breathe out, you'll start to sink. Again, you should not need any air in your BCD for this.
For what it is worth, I have seen weight belts come loose because the latch was not designed for a particular thickness and thus never would "positively" engage the belt (or it was a cheap latch). If this was what happened, you could have latched the belt and it just came loose over the course of the dive (not your fault). It's possible.
Lot's of good info here in these posts!
Your coolness and quick-thinking without panic is admirable!