The issue is not whether or not you were acting reasonably. The issue is whether you have any damages.

In your words, a light crash, a scraped pedal, and a scraped rear derailleur. Why would you even consider getting a refund on the whole bike?

Now, change the fact pattern, had you plummeted down the hill, and but for the lack of rear brakes, crashed into an 18-wheeler and been paralyzed. That's when you explore your reasonableness vs that of the bike shop to apportion fault between you and shop. In this case, no damages, so it's all a non-issue. Save yourself the embarassment of asking for a refund because they didn't flip down the quick-release latch on the rear brake.