Actually this rule is defined in the IG codex and does not require a clarification or rewrite. I wouldn't say it was especially clear, but it is there.
The rule says that "Any Guard Infantry model with a BS of 3 may make a single re-roll of a shooting to hit roll of 1." This answers many of the questions above.
The term "Guard Infantry" (GW's caps) is a rule on page 55 of the codex, where a number of specific unit types are named. Hardened Veterans (HVs) are included in the list but stormtroopers are not.
Arguably, you could buy the skill for the HVs if a priest is attached, though I would argue that you couldn't. I think that when you buy the unit it doens't include any guys with BS3, so they can't have the skill. In any case even if you could buy the skill it would apply only to BS 3 guys, so just the priest. Pretty much a complete waste of time then.
Look out for the term "Guard Infantry" in the doctrine descriptions and where it describes specific units instead. I think there is only one doctrine where there is a description fails to meet any of these, which is Die-Hards. Apparently "Any Imperial Guard Infantry unit or Rough Rider squadron may be given this ability...". "Imperial Guard Infantry" is a term that may mean either any infantry from the imperial guard including stuff like storm troopers (which might be intended in this case). Alternatively, it may be a typo that should say Guard Infantry - but die hards isn't really that amazing anyway so people can get around this by not wasting points on it.
Also, as far as I can tell, Chem inhalers are available without restriction.