What's the role of bread in pickling cucumbers?
There's enormous amount of yeast and lactobacteria on the old bread (mostly sporulated, captured from the air). There's also the same tremendous amount of them readily available on fresh cucumbers.

While the yeast don't thrive much in the brine, lactobacteria are tolerant to salt -- similar process happens in sauerkraut. The difference from sauerkraut here is that getting into cucumbers is a bit harder, and cucumbers don't contain nearly as much nutrients as cabbage. Bread provides lots of starting sugar (and much other tasty stuff) for the bacteria/yeast to survive before they get into the cucumbers and adapt to feeding on something new.

Putting the jar into direct sunlight can get the temperature to 30-35 degrees, which is ideal for growing lactobacteria.

Specifically to your two questions:


Alcohol easily oxygenates into acetic acid (vinegar). This is further supported by presence of various other bacteria (esp. acetobacters).
Bread is an ideal catcher of bacteria&amp;yeast from the air, so even if baking kills everything, a 2-3days old loaf will do. Even by touching an otherwise sterile bread you provide enough microflora to start visible fermentation in a few days. You just filter out the "bad" bugs by putting it into the brine, where only few mentioned species survive.