K
The Turkish Straits are entirely contained within the territory of Constantinople, so moving from Constantinople to Bulgaria does not require crossing the straits. As the straits are contained within a single territory and not at a border between territories, transports are never needed to cross them.
It doesn’t matter what the odds are. Fighters may attack a hostile territory as long as at least one infantry also attacks (all of the units end their move in the same territory, so the infantry satisfies the fighters’ land unit requirement). Of course, if there is only one infantry, all of the fighters will have to be taken as casualties before it can be taken (see page 19 of the rulebook).