Well unless you have a good location for the legendary fort, where it won't be destroyed by 1 gunner the defensive buffs are useless as they are only active when the fort is not destroyed.
Yup. If you visit my base at Red Flag Fleet you will see its inside my walls and all cannons are as close as can be to get the defensive cannon boost.
I'm personally using my fort like you stated, to extend the area where ships can't cross. The indestructible additional troop barrier I find to be much more beneficial than an extra couple percent to defenses, and don't mind if the fort dies. Only the ship portion of the fort is uncrossable, but its extending my shipline an extra couple tiles We'll see what the other pirates bring to the table - cornelius adding stun time to mystic mortars (3.18 seconds total per mortar at 11%) will definitely make me consider a center-fort layout, but I'm not sure I can justify the loss of control it gives me over troop paths. Done right, its significantly easier to get troops into a permanent stun lock with those times, but you need a bit of control over where they attack from, and you lose that by cramming the fort in there.
Yea the abilities that need the Fort to be near the island are useless, the Fort is too big to cram inside our base, pointless really. I didn't even know that the boost only lasts as long as the Fort stands.
Boomer is better for %boost to cannons, it's island wide, not like McKrakens ability that it's just near the Fort that gets the boost, Boomer also gives a boost to the walls. I use McKraken for attacks & Boomer for defence.