If this is a heat pump the defrost is not working. You can turn off power and melt with hair dryer. Then you will need to find why it is not going into defrost. You can really melt with any heat source, but I recommend being careful and using open flame.
Wow, I think the deicer would work some, but in thin layers at a time.
Iit would still hasten the project to apply heat.
Do you have electric back up heat in your system
No it will not cause trouble. But yes it will smell, and yes everything is normal with that. There is a fine coating of dust on the elments and this will cook off. It is smelly but dissapates pretty fast.
I wish you luck and will remain available for follow up for you.