1. The flat is mitigated, really. I guess there's a little latex plug blocking the leak, but in my case pulling the tubes out and pumping without a tire opened up pinpricks I didn't know I had.
2. Yes. (although it probably depends on the size of the pinprick/cut/tear). Sealant is generally not as messy as Slime. Usually it has the consistency of cream, with some kind of added chunkulant (glitter, rubbersand, fibers) to help plug holes. If you are holding a tube with the puncture upwards to patch it, any sealant inside will tend to drain away from the hole. Wipe the surface dry if necessary, and then it is no problem to sand and vulcanized-glue patch. In my experience, even though sealant that might come through the hole to contact the glue and patch, it does not weaken the bond or work the patch off.