For example, will the bleed damage from fighting form benefit from cadence’s 500% weapon damage?
No, as Fighting Form does not add Bleed damage to your weapon, it adds it to Cadence alongside the Physical damage on the base skill.
Flat damage from other sources, including Deadly Momentum will affect Cadence, but only after you receive the buff. This means your first Cadence will not have the Deadly Momentum flat damage included in the %Weapon Damage
Nope, it’s not a buff like deadly momentum. Same for laceration, internal traum on FW, CoS
Thanks for the replies guys. I just wanted to be doubly sure. 
An easy way to remember it is: weapon damage is weapon damage, and skill damage is skill damage.
Even if a skill has both WD and SD such as Fire Strike, the latter is just counted as SD no different from SD on skills like Trozan’s Sky Shard.
There are some exceptions such as Deadly Momentum and Soul Harvest, but these abilities specifically mention that they affect all attacks while active.
So, if I am playing a PURE internal trauma build, is there a reason to put more than 1 point in blade arc? I’ve already maxed laceration and I thought BA’s WD modifier affects laceration’s IT damage.
I wouldn’t build BA if I were you as it does bleed damage.
But the increase in %WD will amplify damage from IT sources like deadly momentum, items, devotions, etc.
For IT, best would be something like 1 pt. BA and max laceration. The WD increase in maxing BA won’t help in terms of trauma cause you’ll have maxed forcewave anyway.