I know I'm not much help . To me it reads as if they just get deployed out as per normal.
This is what I'm running off:
"or up to 12" in , along either side edge"
I'm assuming "in" means in from the side edge, some S3 infantry would have problems even in theory manoeuvrings in 12", also for example fast attack specifies on maneuver orders so I think it would have been included.
Interestingly enough that last sentence to me seems a little ambiguous.
"or up to 12" in , along either side edge"
Does it mean 12" in from any were on the side edge. I'm pretty sure that's how Steve's boy's hurt you bad as they deployed right on your left flank.
or
Does it mean 12" up from the standard deployment edge alone each side edge ie you get to come in from the side but not that far up on the side?
It is a fairly low cost Tactical Advantage, fast advance is twice the cost and doesn't seem half as effective to me if the first interpretation is accurate.
Just my thoughts.