> In some plastics the monomer is toxic, but the polymerized form is safe.

This is common. Isocyanates are a common example — isocyanate monomers are nasty, and the very light ones are very nasty. They’re used to make polyurethane, polyurea and such, which are quite nontoxic in polymerized form.

In applications where the unreacted isocyanates are used by anyone other than professionals (e.g. two component varnishes), the manufacturer may go out of their way to use more expensive but less toxic variants.

Correct, PMMA is completely harmless. MMA is an incredibly common adhesive, and is in probably a dozen things in the room you are sitting in in its polymerized form.