Anytime you're dealing with a flight, you have a possibility (even a probability) of sparks.
Perhaps they made the choice partly due to a public perception of the safety issue as that scene from 1938 in NJ of the burning Hindendburg is indellibly etched into the public's collective consciousness. (Witness the lack of interest or R/D $$$ in hydrogen-powered cars). The actual reason for the incendiary explosion of the Hindenburg is still hotly (no pun intended) contested. I think the reason you list in your explanation is the current scientific theory. However, the mfrs chose helium for lifting the craft for a reason. I'll bet it's about cost and safety concerns, if not marketability to the public.
Here's a quote about comparisons:
"Hydrogen Arguments. While helium is exceedingly light as compared with air, it is somewhat heavier than hydrogen. The total lift of a helium-filled dirigible is accordingly some 10% less than that of the hydrogen-filled airship. The difference does not appear important at first sight, but the total lift of the gas carries the structure, the motors and the crew. It is only the last 20% or so that is available for carrying fuel, and hence a difference of 10% in the gross lift may spell a difference of 50% in the fuel-carrying capacity. On long-distance flights this difference is vital.
Nor is the danger of fire totally eliminated with the use of helium; the gas-tanks and the fuel system generally are still vulnerable. But when a ship is properly designed and carefully handled, the danger of fire is comparatively small, even with hydrogen.
Another strong argument of hydrogen partisans is the fact that owing to the minute quantities of helium found in the natural gas at its source, an extremely expensive system of fractional distillation is necessary and the cost will always remain excessive.
General View. For a number of years the Navy has held that our helium monopoly meant supremacy in the air as far as dirigibles were concerned. But recently the attitude of the Bureau of Aeronautics has changed and its officers in various public utterances have advocated a return to hydrogen.
A strong technical case could be worked up for either side."
Read more: http://www.time.com/time/magazine/article/0,9171,786149-2,00.html#ixzz1ABWIGsDx"
FWIW, I'd be interested in a ride in that helium-powered craft, whereas I wouldn't even consider a ride in a hydrogen-powered vehicle of any sort.