Will stop leak temporarily help with a blown heater core

No reason why it shouldn't. You could also cap off the heater lines and adjust coolant level until you get a new heater core.

It might, but it may or may not cause problems elsewhere. The best fix is replacing the bad heater core. I would never use the stop leak products in my own vehicle, but many people have good luck with them.

Alumaseal has worked great for me when I have needed it.