I believe Fujitora was sent after Hancock, reasons being:
1. He was in Marejois, very close to Amazon Lily.
2. Amazon Lily is very close to the WG establishments so better send an admiral to clean it up
3. Fujitora is blind so Hancock's devil fruit power would be ironic
4. Amazon Lily located near calm belt, and when Fuji was talking with Akainu after Reverie we saw a sea king next to his ship
It's definitely possible, and for the same reasons I spoke of Fujitora in my First message, but:
- if Fujitora were sent to Boa, why wouldn't say It clearly and only show Koby? For the suspense only?
- we know that even if Boa were the strongest she could be, she wouldn't stand a chance against an Admiral, much less against Fujitora who is a Natural counter to her. If Fuji beats her, nothing really changes: it's only given.
On the other hand, if Koby does it not only it's big news, but Koby gains an entirely new role;
- imho, it was long implied that, after the downfall of Smoker, Koby Is the actual new generation Marine: not only Garp disciple, but nicknamed "Hero" like him, he has promised to fight with Luffy in the NW, it's a SWORD member, etc...
- last but not least, Oda likes to "surprise" the readers: as I said before, an Admiral beating Boa it's predictable, "normal", the unpredictable would be Boa's victory or at least her escape; on the contrary, Koby's victory would be much more of a plot twist.