It's additive and examples 1, 2 and 4 are correct.
Thank you for the clarification.
Am I right in assuming this additive game mechanic makes damage enhancements less advantageous for brutes and thus acc/recharge/end become better?
Correct, at least for early levels. Slotting for Damage is far from disadvantegous though, you should definitely slot for Damage. But you can skimp on it in favor of acc/end in early levels.
On my level 32 Dark/Invuln Brute I have just about 60% worth of damage enhancementing on my attacks, the rest is a good mix of acc/rchg/endredux so I can SMASH more often and stay in battle longer, resulting in MOAR DAMAGE! 
I've recently made a db/wp brute and it's been cutting and slicing happily past the level 30 mark. I've read lots of brute guides but I was unable to find precise info about how fury works with other damage multipliers; Whether they are additive or multiplicative?
I'll use examples since my questions might be clearer that way, which statements are true/false:
Example Power does 100 base damage.
1) At full fury it does 300.
2) At full fury plus 25% inspiration it does 100*3+25 = 325
3) At full fury plus 25% inspiration it does (100+25)*3 = 375
4) Slotted with +75% damage and at full fury it does 100*3+75 = 375
5) Slotted with +75% damage and at full fury it does (100+75)*3 = 525
Any clarification would be greatly appreciated since whether 4 or 5 is correct has huge impact on damage slotting.