Is this natural?

[overweightninja]

Is there anything that can make the diplomacy better (other than the force diplomacy mod)?

I've heard lowering the campaign difficulty to Hard or Medium can help relax things a little, the military and diplomacy AI are apparently nothing to do with eachother though so I'm not sure how much difference it'll make to the diplomacy side of things.

Cant their attitude towards Mak be toned down in descr_strat?

The only thing I can think of would be changing the tags for factions reading things like "trader napoleon, balanced ceaser etc", I however have no idea how much of an effect if any these would have (never looked into what that actually do but I assume it's the "personality" of the AI).
Setting factions to be allies with eachother at the start of the game is possible and would probably help (for a limited amount of time anyway), however that would have to be something you take on yourself as I imagine the current diplomatic relations are designed to be historically accurate.

EDIT

Of course, it can, but the point of this attitude is to encourage Pyrrhos the *...* to actually attack and seize Pella for a chance.

Yes, the hellenes knocking the crap out of eachother is always the way to go :D

Cheers