To negotiate the settlement you'll have to wait until theres no high ranked non-bribable characters in it. You are probably trying to negotiate surender with a royal family heir, and they usualy cant be bribed/intimidated.
Just wait for him to leave that town, and try again. You should be fine then
As for Egyptian chariots they are over-powered and have been toned down in previous patches and mods. The best way to get them is to take them down from a distance with archers or skirmishers.
And if you dont know the Rome total war golden unit exploit yet, here it is :
Your army should only be 1 heir unit and the rest should be wardogs... all of them...
Wardogs are overpowered like hell, they can take down entire capitals filled with top rank units without a single casuality

So try that out