If a buff last 3 minutes we could become too Overpowered? Excuse me but i think there is a big mistake here. A SAM With tachi Fudo against a MOB of high level could reach damage of 10+k without using mythic or relic or empy. To reach a similar amount of damage with blu i must have Tizona with tons and tons of equip, and even in this case is still difficult.

Diffusion and duration of buff is an issue? well really easy to solve:

Animal ConcentrationDuration 1m or till next buff spell / Recast 7s Double the cost of your next buff spell to grant a bonus on duration effect / Impossible to stack with Diffusion and Unbridled Wisdom/Learning

It will work similar to pianissimo with the recast time to allow us to get buffed
i don't think for many spells that making them during twice is enough on the other hand buffs like refresh or haste could reach a peak of 10m of duration, if you think it well there will be no difference since the ratio MP/Duration will still be the same.

Nature's Meditation Boost ->3M
Orcis Counterstance Boost ->4M
Cocoon boost -> 3M
Barrier Tusk ->4M

Battery Charge ->10M
Haste's Spells -> 10M

Here we are, we have solved the problem of BLU magic Duration and diffusion.