Need help with magic Mag

Does this make you convulse all over?

I doubt SG will swing much, but yes, Mg could. considering the guideline is not to raise it by more than 100ppm/day, I'd assuming lowering holds the same rule. So I'd do a quick calc to see how much 1100ppm Mg water could go into your water volume to drop only 100ppm. Might be a larger quantity than you'd think, so it may not be too much of a concern. Tell me your water volume and I can run the calc.

Agree that once/month would minimize swings, and agree with the methodology, but I'd probably do it every 2 weeks to get it over with sooner.

Yes, continue there. No, corals don't use Mg the same way they use Ca (to my knowledge). Also, keeping your Ca constant should help, as some Mg will bind to the Ca as it tries to precipitate.