If your husband poured barsleak in the crankcase, it probably stopped up the oil pickup screen. It is supposed to be poured in the radiator. The engine is probably ruined if you have driven it with the oil light on.
Have someone drain out the oil and replace the filter, now! Do not run the engine! This is not good for the engine!
The engine will knock.
Either it will not run at all, or run very poorly. Either is likely to do severe damage to the engine.
It will stall the engine
Poured on the outside - no. Poured in the gas tank - depends on how much fuel you had inside. Almost empty - odds are the engine won't run. Mostly fuel - it'll be fine.
When antifreeze is poured into your oil, it weakens your oil cause it to break down prematurely and can cause your engine to fail by spinning an engine bearing and even smoke. This is also a sign of a blown head gasket.
Fire needs oxygen to continue burning. Water cuts of the supply of oxygen and hence the fire stops.
The best quick fix is to replace it as there is no quick fix. There are products that can be poured into the engine that claim to fix bad head gaskets. That's more of a "let's see if we can make this even more expensive" fix.
any thickness of concrete can be poured however if it contains an aggregate you would be limited to the diameter of the stone. you may also replace the water component with a latex additive that will substantially improve the strength.
The water takes heat in the form of latent heat at its boiling point to convert into vapour. this puts the fire down.
Sand is heavy enough to push out all the air from the burning material. If a fire is starved of oxygen - it cannot burn.
Draining the excess will solve the problem.