I've always seen/played/understood that things like DD leader, BB Leader, and Flotilla leader are init bonuses on top of flag. So I get +2 for PoW, then +3 for Jamaica. I think that was an officially clarified thing at some point because I've never seen nor heard of anyone doing it differently. I know flags don't stack, so I don't get +1 for DoY, and +2 for PoW...but like other bonus granting SA's, as long as they are different SA's I believe the bonuses stack up.
I forget what nation it is, but one of the nations can pull off a +9 init I think. And open nation (with no point restriction): +2 flag, +3 Flotilla Leader, +3 Carrier Leader, +3 Landing Leader, +1 Cruiser Leader, +1 BB Leader, +1 DD Leader, +1 Picket line (Or some SA from TeamDeckB) = +15 Init!!!