Hector's true plan is...what Manny thinks it is. There are two things that make Manny believe that Hector is trying to buy his way out of the Land of the Dead. One is that toward the end, in the cutscene where Hector takes Manny back to his old DOD office, Hector very clearly says that he needs the tickets to get himself out. The scheme wouldn't work, but he thinks that it will. The other thing actually happens toward the beginning, when Manny is brought into the LSA, Salvador says generically that there are people who try to buy their way into heaven. That statement is a bit of foreshadowing. What Sal says about "some", Hector applies to himself. So when Manny says what he does in the greenhouse, he isn't guessing. It's what Hector told him.
Domino has other ideas about what the plan is. He doesn't want to leave and he attributes his motives to Hector. There's a contradiction there which can be resolved by assuming that Hector didn't let Domino in on his private motives. I used that in the novel and VampireNaomi might be thinking of that; but it's a pretty obvious rationalization and any number of other people could have made it, too.
As for the purpose of the greenhouse (and the meadow): if you examine the water tank, Manny wonders if Hector sees the greenhouse as a monument or a memorial (or words to that effect). Hector is basically a very sick man. The greenhouse and meadow are really no different than any serial killer (the so-called Craiglist Killer being only the latest example) who collects tokens from his victims. I think only an expert in criminal psychology could really explain the why of that.
(Obviously, like malaria, I've come back.)