I don't think you can make an ally or ceasefire when it is 1 v 1. Even in larger games unless you set the allied victory conditions to allow more than one player to win, the AI will declare war on you when you are the last one left. Though why would you want a ceasefire in 1 v 1? Wouldn't it be better to disable the AI player entirely making you the only one in the system?

imo its kind of ridiculous how easily the ai a)places so much importance on missions and b)becomes friendly with you so easily, in late game with a at least 3 other factions i usually end up at peace with all except one, who i defeat, simply by getting the first 2 or 3 relationship uprades and not attacking them, which is pretty easy since theyre far away

if i fail a mission because i wasn't paying attention and its too late to reject it, my relationship with the ai who offered the mission drops 2-4 points (or more), even if its to attack my trusted ally who i have 10+ relationship with

The AI will not make alliances in a 1v1, but it's actually very easy to make friends with them in larger scenarios.  You can make AI's very happy by completing missions (usually involving killing another AI's units) and after two or three missions they will make peace.

If you don't like the mission system (like aaa11) you can play with locked teams.  This is how multiplayer is usually played, where teams are set in stone.  You can still form pacts with your allies, but the allies you get are determined at the start of the game and can never change.

I was playing an 8 player FFA and six of the AIs gave me missions to kill this one other AI.  So I knocked him out in 20-30 minutes and everyone else offered peace treaties