For the conflict where you want the players to realize the Fomor are not allied with The Fisherman I would just do that as storytelling. No real reason to bring mechanics into it unless it's something you want them to have the possibility of discovering, but not required to. If you want it in mechanics then having them roll things like Alertness (or maybe Investigation if they're actively discerning who's on who's side) or you could even use something like an Empathy roll for "Reading People" to help them to determine who's who in the fight.
For the chase I would probably do it as an Extended Contest (YS pg. 193-194). You could do this as Cat & Mouse, but it may be more fun to throw some extra stuff in to create a sort of Custom Skill Challenge.
Example
Someone begins chasing The Fisherman. The Extended Contest will be a variant of Cat & Mouse with five opposed rolls.
The Fisherman gets a +4 after his first roll.
The PC gets a +3. Current total +1.
The Fisherman gets a +2 after his first roll.
The PC gets a +4. Current total -1.
The Fisherman rolls a +3 Might to throw a trash can at the PC.
The Fisherman gets a +3 after his first roll.
The PC gets a +2 Athletics and is hit by the trash can.
The PC gets a +2, a +3 roll -1 from the trash can hit. Current total 0.
The Fisherman gets a +2 after his first roll.
The PC gets a +4. Current total -2.
The Fisherman bobs and weaves through some traffic using a +3 Athletics.
The Fisherman gets a +3 after his first roll.
The PC gets a +4 Athletics to dodge the traffic too.
The PC gets a +4, a +3 roll +1 from catching up during the traffic dodging.
Final Total: -3 (PC Wins and catches up to The Fisherman!)
Obviously you'd want this to be very narrative heavy, and likely more than just one PC would be making rolls. This is just the general idea of what I mean.
For stat blocking The Fisherman himself I'll need a bit more time. I'm heading to class.
Hope this stuff helps!