I might be wrong here, but a detachment gaining Steadfast from an unengaged regimental unit is a pretty creative interpretation. Steadfast isn't a rule you have by default. It applies to units at the end of a combat phase. A unit can't be Steadfast if it isn't in combat (well, Stubborn units can, but that's a different rule altogether). Thus, an unengaged regimental unit cannot pass Steadfast to its detachments because it doesn't have the rule.
On the subject of flaming (and fabulouth (said in a high note and a lisp at the end)) handgunners or crossbows. I like the idea but my main concern is that they don't pack the punch to wound the monster in the first place. The main purpose of this tactic is to combat hydras, right? if we start with 10 crossbows, BS 3, shooting at long range (usually the case), 3.3333... shots will hit. S4 vs T5 means that normally 1 of those shots will wound and then the hydra has a 6+ Sv. Sure, we only need to put one wound on it, but it's still a gamble. T5 is really the edge of where this tactic is at all possible. Using it to combat trolls or regenerating ogres is much more viable though.