c) AI has very big alliance advantage - how come that e.g. Alexander or Darius or English are allied with most city states, having only a few cities themselves? That is impossible to achieve for a normal player;
Other than Greece/Siam, it isn't too difficult to secure allies. Start paying attention to the quests they give, and keep a couple units out scouting. Clearing a barb camp with a city-state worker (and giving the worker back) is a guaranteed push into alliance. From there, you only need ~10 gold per turn to maintain relations. Selling a luxury to another Civ will net you 8 GPT, which basically covers it.
I've had a number of games where I had 4+ solid alliances with city-states by T100.