Intel -- Do you play with it on or off?

[capnq]

I always leave it on for solo games.

Very few people use intel in multiplayer games, because it tends to be all or nothing. Either everyone has rough parity, and virtually no attacks get through defenses, or empires that have fallen behind get manhandled by the leaders in the way you describe.

Artaud said: I never pay much attention to [intel] until my enemies are pounding me with sabotage, stirring unrest on my planets and stealing my resources.

This is your first mistake. Unless you are roleplaying a race that wouldn't do so, you should start researching Applied Intelligence as soon as your first First Contact at the latest, and start building at least a few Intelligence Centers and running Counterintelligence projects as soon as possible.

EVERY other empire is at war with me, even though I'm ranked 9th. None of the AI empires is at war with any of the other AI empires, which seems very odd

If you are at war with an empire, you cannot see what treaties it has unless you have an ally who has a treaty with your opponent, or successfully run an intel op that reveals them. With every other empire at war with you, your treaty grid should be empty except for your war states.