Your post begins by stating an impossible situation :
« with teams locked », as you have declared ... there cannot be any form of diplomacy !
Diplomacy is only possible if teams are UNLOCKED in the game setup options.
If you want to be certain that the comps will not gang-up on you, you must define LOCKED teams before starting the game AND you must put a single A.I. per different « team » when you select your computer adversaries :
For example, if you are playing against 8 comps, you would have NINE locked teams (checking that to each pseudo-team # is associated only one player).
Then, with LOCKED teams, you would have NO diplomacy, and the comps would not usually gang-up on you.
That's why I find your post's situation very odd : « with teams locked », as you have stated in your OP, you should NOT have had any form of diplomacy going on !
One hypothesis to explain your odd situation is that you defined « teams locked » before starting the game, but the option was broken if you reloaded a save-game.
That's because there is a bug (fixed in the upcoming 1.04 patch) : when you load a saved game, the options are reset to default, where teams are unlocked ! (The present solution is to re-change those default options and re-accept your changes just before you're going to reload.)
In conclusion, if you don't want the comps to gang-up against you, follow my instructions : but then, your game will have NO diplomacy whatsoever ... which is not a great loss, because I somewhat agree with you that the A.I. missions are mostly annoying tasks.
An Ironclad dev has stated, a few weeks ago, that Diplomacy will receive a major overhaul for the game's first expansion. On the other hand, Stardock & Ironclad will continue to publish patches & updates, and Diplomacy will then probably be tweaked.
The very end of your OP, where you declare that the game is « rubbish » and that one should not buy it, is extremely unfair ... especially if you take into account what I have just explained to you. This game is very fun to play ... if you know what type of game setup to define, to suit your tastes.
Lock the teams, and keep them locked !