yep. I doubt they cared about the drug supply thing considering the benefits they were getting out of the arrangement. Well, the benefits they would have gotten had Picard not sabotaged the operation and turned the Son'a into outright allies of the Dominion rather than merely suppliers.
Becuase a finite source of magic youth particles really helps with getting shot by a weapon set to kill or being exploded
Meanwhile there's really nothing to stop the So'na from sharing their particles with the Dominion meaning, Yay advantage nullification.
it would help by doubling the life-span of your available troops, and it would heal most battlefield injuries, and heal conditions which would have prevented others from joining the war effort in the first place. The fact that you brought up about the only scenario in which they WOULDN'T help doesn't exactly negate my argument.
As for your second point, I would hope that the UFP would be smart enough to conclude a deal whereby in exchange for giving the Son'a access to the planet, they would agree not to share the particles with the Dominion or their allies.