First of all, if you don't add any heat, you leave behind saline. So your "higher yield" is just saline that didn't evaporate but instead dried into flakes.
And yes, saline dries like that. Pour out contact solution and let it air dry. You'll have quite a bit left behind. Contact solution is saline, btw.
If you leave the microwave on the lowest power setting, it's not enough heat to damage the ketamine. You simply need a very large plate because some of it will splatter. If you use a big plate, you'll catch it all.
I know there are sooooo many people that diss the microwave thing. However when I was a ketamine dealer back before the stuff was scheduled, I had people tell me all the time that somehow my ketamine was better than everyone elses. When they asked my cooking method my answer was - power setting 1 on the microwave.
Sure, my yeilds were smaller... but who cares? Are you worried about weight or ketamine?