Q1: IEEE 1584 better models switchgear up to 15 kV. This is clearly stated to some degree in NESC. However, it falls down completely above 15 kV at which point you are stuck with ArcPro and the multipliers they recommend. The multipliers are simply geometric assumptions.
Q2: Arc flash CAN occur even down to 208 V:
http://www.efcog.org/wg/esh_es/events/ESSG_Fall_11_Meeting/presentations/10-208V%20arc%20flash%20calcs%20efcog%20fianl%2010-12.pdf As of right now NESC references utility industry tests and allows everything up to 250 V to be rated 4 cal/cm^2 which still requires arc flash PPE but it is relatively minor (AR shirts and pants, ear plugs, hard hat, safety shoes, leather or rubber gloves). At 208 V or less, IEEE 1584 suggests that the arc flash hazard is 1.2 cal/cm^2 but this is purely a suggestion. No actual test work was done and it reads more like an anecdotal suggestion. Actual testing (not yet published) suggests otherwise. See the reference above which suggests that a lower cutoff of 45 kVA should be used. The test work seems to suggest that arc flash is possible and exceeds 1.2 cal/cm^2 for 208 V or higher cirucits down to around 4.5 kA. I have not seen similar information for 240 V or 120 V yet.
So your choice is to either follow the IEEE 1584 suggestion (not a rule), or follow the NESC rule with mild arc flash requirements, or perhaps get the research papers and follow IEEE 1584 but lower the standard to 4.5 kA short circuit or 45 kVA transformers.
Above 208 V I haven't seen much in the way of test work. IEEE 1584 has exactly ONE test at 240 V that it relies on for data which really isn't enouhg data to go on.