You are rewording the NEC language and thus changing the meaning. Under normal conditions does not translate to always present or always anticipated to be present. That may be the case but is not necessarily true. Zone classification for instance makes the distinction between Zone 0 (always present) and Zone 1 (can exist under normal operations). In our classic system both are C1D1.Division 1 the ignitable concentration of flammable gases, vapors or liquids can exist all of the time and is anticipated that they are there all of the time. A higher design safety factor must be in place.
Division II the ignitable concentration of flammable gases, vapors or liquids are not likely to be present but intermittently they can be. Lower design safety factor.
Try reading the question as a two separate parts; A location in which hazardous concentrations of flammable vapors exist intermittently is designated as
Division 1 ignitable concentration always anticipated to be present. Higher safety factor required.
Division 2 vapors should never be present, only under intermittent conditions. Lower safety factor required.
Both scenarios are under normal conditions; Division 1 anticipated to be there all of the time, Division 2 not anticipated to be present all of the time, except intermittently.
"Under abnormal conditions" in no way translates to "intermittent". In fact I would say just the opposite. Abnormal condition would be be a relief valve lifting, a spill, valve stem leaking, etc. You last statement is a direct contradiction of the NEC in that you state Div 2 is under "Normal" conditions. The code specifically states that it is under "Abnormal" conditions.
In the end it is up a to PE to make that determination using the appropriate API and IEEE standards. There is no way that I as an electrician or inspector would take on that responsibility or liability.