In Star Trek: Insurrection did the reasoning make sense?

Question

Why didn't the Son'a want to share the planet with the Ba'ku? The admiral seemed to agree the only option was to move the Ba'ku?

Answer 1

The Son'a are too far gone for the normal affects of the planet to work for them. They are also the only ones with the technology to be able to extract special radiation.

The planet is within Federation space, so the Son'a need the cooperation of the Federation, if they don't want to be chased across the galaxy.

The Admiral was getting on in years, and was starting to fear death. He wanted to live forever.

Answer 2

The Son'a were exiled from the Ba'ku planet after their hostile takeover failed 100 years previous. The Son'a were using the Federation to take revenge on the Ba'ku. This seems to make sense.

Admiral Dougherty and the Federation council had no knowledge of that event, or the relationship between the Son'a and the Ba'ku.

Additionally, Riker and Troi hang a lampshade (TVTropes) on the strangeness of the Federation cooperating with the Son'a about 15 minutes in:

Riker: Half century ago, they conquered two primitive races.. The Tarlac, and the Alora, and then integrated them into their culture as a labor class. Look at this: The Son'a are known to have produced mass quantities of the narcotic Ketracel-white. Their ships are rumoured to be equipped with isolitic subspace weapons outlawed by the second Khitomer Accords.
Troi: Why would be be involved with these people?
Riker: Good question.

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About the "duck blind" itself, Dougherty says:

Our procedures were in place to protect the planet's population from unnecessary risk.

The plan was to use the holoship to remove the Ba'ku without their knowledge that they had ever been moved. A similar concealment was used for observation in TNG 3x04, Who Watches The Watchers, and implies that the Federation did not think the Ba'ku were technologically advanced. Therefore, they couldn't simply go in and negotiate without breaking the Prime Directive; to gain access to the metaphasic radiation, the Ba'ku would have to be relocated.

As to why they were so desperate: This movie takes place during the Dominion War, at a point when the Federation is losing. Having that metaphasic radiation available in sick bays would have been a great boon, and having the Son'a on the Federation's side would mean they're no longer creating Ketracel-white for the enemy Jem'hadar, who are genetically addicted to the substance.

So the Federation made a deal with the devil, in order to try and gain an upper hand against an even bigger devil. So yes; this reasoning also makes sense.

Answer 3

It's psychological.

First, the Son'a were from Ba'ku originally, and were even related to the people still living on the planet, so they knew the people who were there.

Second, the Son'a had left and had grown angry. Anger and hate changes and warps people. It leads them to despise the negative in their own self and to blame all that on others, or to project their own faults on other people, whether that's valid or not.

The Son'a hated the people on Ba'ku - mostly because the Son'a had fallen from paradise and were angry about that. They were even angrier of the ones who stayed there, because they were right -- and when you're angry, you can't admit you're wrong, and will blame those who are, in truth, right, for the mess you're in.

They were driven by anger and that made them hate their relatives who were doing well, so they wanted to take their anger out on the peaceful Ba'Ku. They wanted to remove them and to take it all for themselves.

As for the Admiral, well, if you offer someone immortality, then say, "But to get it, you have to help us do this bad thing," unfortunately, there's a lot of people that will just follow along.

I wish I could give you a "Treknical" explanation, but it boils down to anger and self-loathing that is hidden by pretending it's someone else they really hate.