Odo would not have cooperated if he knew what it was going to be used for, and he probably would have asked. Maybe Starfleet Medical collected a sample during Odo's exam, or asked Bashir to send a little bit of his samples. Also possible Layton or one of his people made something that looked a bit like it and just made sure no unbiased person had time to examine it closely.
Some or all of the samples Section 31 needed could have been provided by Dr. Mora.
Considering the Link is a giant ocean of goo, and each Changeling comes out using an unknown amount of goo, could a single Changeling actually split itself into 2 or more beings?
My assumption would be that he took a sample of Odo's (cooperatively initially) and from there used some to develop the mutagenic virus and some to con the test
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