It will burn extremely hot and create black toxic smoke. Never burn used oil. Take it to any auto parts store for recycling.
What happens when your heart stops? The motor would seize, thus no more motor!
it would be very slippery. But also if there is a fire by or a lightning storm it can catch on real fire.
don't try it
Type B
It would probably clean out the gunk from the motor. It could cause the oil to lose its viscosity and cause a bearing to fail.
oil leak, worn rings burning oil if only a quart or two between oil changes not too much to worrie about on some engines but would have checked out.
I would use a synthetic 20/50 motor oil
Yes, you can not only switch you can mix synthetic with conventional oil.
Nothing terrible will happen but the old oil in the filter will mix with the new. I would recommend you change the oil and oil filter again. This will insure that you will have clean oil throughout the motor.
Not much unless you live in a cold climate then it may get harder to start when it gets cold.
motor oil, of any kind, would kill most plants
If the seals between oil and fuel has been damaged then mixing will happen, take the motor to a qualified repairer