Personally I agree with you that heavy braking with only one hand on the bars is too dangerous. Yes, it can be done, but if it's on your commute I think that the risk of something going wrong some day (what if you hit a pothole, for example - there are soon going to be a lot fewer daylight hours), and the possible consequences of something going wrong, are too high for you to do this routinely.

Choosing a different route, as others have suggested, is the best alternative if it is feasible. The only other alternative I see is to brake to a speed that's slow enough, and do this early enough, that you can stop braking (or, at least, brake gently enough that you can maintain good control) long enough to signal, then get back on your brakes to make the turn. I've done that a few times and I'm still here, though of course I have no other way to gauge the effectiveness of this strategy.

As a motorist I personally would rather be stuck behind a cyclist who was going a bit slower and informed me what was going on, rather than someone going faster who then did something unexpected - like taking a left in the middle of a descent. There are probably others who don't agree, however, and would rather you just "get out of the way" as quickly as possible. You know your situation and your area best, so you would be the best judge as to which strategy is best.