@Trusty
Just a quick suggestion/idea, while in melee mode we could use a "fire/water" approach. Left click is stab, right click is a smack with the butt of the rifle. To defend yourself, you would need to press the correct "attack" button to parry their blow. No blocking, just parrying to the side or something. It would reward skill for the attackers while opening up a window for the teamwork aspect. Keep the butt strike a two hit kill, and we have a basic and fun melee system.
To expand on the teamwork aspect I mentioned, we could make the stab a "heavy attack" that can't be immediately used once you've parried. The rifle butt would be the only option after a parry, meaning a slip up or a parry isn't the end for the attacker. This also means that a bayonet charge would allow someone to save a friend that is locked in a fight with an attacker.