Yep, thats the other trick with soldering, not screwin up what you got already
Try to find low temp solder (I think they make some that will flow around 400 deg F now). You can wrap the blade in a wet towel, or have it submerged in water too. Just remember to watch the colors on the steel. If the color moves up onto the blade and gets darker than what you tempered too, you've gotten it too hot.
The funny thing is that when soldering you probably got the knife too hot and made it soft, which I would have thought would make the knife bend easier. Did it kink right at the gaurd and then break like it was overly soft?
You didn't quench it in anything right after soldering did you? Might have hardened it. Might have been ok if you tempered again after soldering.
I haven't gotten into the differential heat treat thing yet. I mostly make blades under 5" and don't really find it necessary for them. Nothing against it, just not something I've tried.
Bending to destruction is a great test though for several reasons. The best I think, is that you can look at the grain structure. The finer the better. With your differential heat treat you can probably see variation in size across the blade. You want to keep it from being too coarse because that affects the strength. A peice quenched from forging temp will have a very coarse grain structure compared to a peice that has been properly normalized and quenched for example. They should have about the same hardness, but the coarse peice will snap MUCH easier.
It also lets you test the flexibilty of your knives like you've done and see if you have them where you want them. Personally I like a stiff blade and would rather have one try to hold its shape, other folks want to be able to lay into one and bend it back when they're done. All personal preference and something to tweak to your liking.
