What other safety mechanisms - besides blanking and shutting off the valves - were in effect or should have been in effect?

There was a Gentleman in earlier correspondence that suggested blanking is unwise and I agree, for I have inspected boilers and burner fronts for years. Double shut off valves is the proper way to secure a gas line with venting. This bit map schematic -below- is an approximation of what I would expect as a minimum arrangement for the gas piping for igniters. This schematic suggests there are safe guards of at least two levels. They are Block valves and Vents.  If either of the block valves leaked by, then gas pressure could build-up and pour into the furnace only if the vents are plugged. If the work order required the workers to replace the IHSSV (Igniter header safety shut-off valve) then the gas line header in the steam/power plant should have been shut down. This line also has a safety vent.

I would like to make these generalized comments: The NFPA and insurance companies writing property and casualty policies set the pace for the burner management system. Furthermore, Jurisdictions use the NFPA or a mirror image of it. As a rule, most Fire Insurance inspectors are not boiler people; consequently, the fire inspector may over look the inspection and testing of the burner management system. The boiler inspector like the National Board of Boiler and Pressure Vessel Inspectors are interested only in the pressure retaining components and not the burner front. The Jurisdiction, where the insurance boiler inspector has his commission, is interested in the burner management and pressure retaining components of the boiler (true most jurisdictions). The insurance fire inspector is interested in the furnace explosion aspects that encompasses the burner management system and burner front.

Can anyone identify any companies who make gas valves such as may have been used in the Rouge power plant?

I would like to say this about safety block valves (more applicable to question 2): They are FM, UL, etc... tested or they met those agencies' criteria. This decreases the probability off the shelve faulty block valves because of the serious nature in which they are used. They are difficult to tamper with, but not impossible. They may leak like any valve after a number of cycles.