Re the hydrogen mixed versus unmixed...
I, along with a few other engineers, used to run some science shows for elementary students. My part of the educational program was the "Fire" section. I ended that section by poking three balloons with a candle taped to the end of a yardstick. The first was filled with helium and put the candle out when popped. The second balloon was filled with pure hydrogen. When popped, this gave off a nice fireball with a mild noise.
The third balloon I filled with a stochiometric mixture of H2 and O2. This balloon detonated, letting out quite a shockwave and deafening boom.
So H2 definitely goes BOOM when pre-mixed with O2. However, a pipeline leak will have H2 shooting out at a high velocity. Flame propagation speed of H2 is only 1.5-3 m/s (I see various values for this), meaning a leaking pipeline will generate a constant flame if ignited.
The problem, of course, comes from the initial ignition. If the leak doesn't end up self-igniting, you certainly have a situation where an explosion will happen.