Thank you for all your help...
I was finally able to remove it by prying between the manifold and the bracket on the pipe (the one used to fix it against the manifold).
What I found is that someone had already changed the gaskets, and apparently had overtightened the screws crushing the plastic part around the coolant passages. At least that's what it looks like. The interesting thing is that I found that the right rubber seal had disintegrated, shredded in pieces, and there was a large amount of oily sludge right underneath it, above the transmission. I hope that was the main oil leak. THANK YOU all for talking me into changing the lower intake manifold!
Now I need to clean the mating surfaces, but it looks like it going to take some time... any trick/magical product for doing this?
Also: there's some sort of cover on the right of the lower intake manifold, held in place with three bolts, and it looks like there's a coolant passage behind it. Has anyone had a leak around this cover?