Subaru Problems? Ask a Mechanic for Answers ASAP
This doesn't sound like good news I am sorry to say. This sounds like the car might have damage to the head gasket and also inside the engine. Not that the oil and coolant is mixing, but the misfire/loss of power could be a result of coolant burning instead of fuel. This will cause the white smoke, and the overheating. With the car being so low on oil, the rattling sound could be a result of something inside the engine coming apart.
I am not sure what you would like to do, or can do on it yourself. Honestly it probably shouldn't be driven and should be towed to a garage unless the head gaskets or an engine swap is something you can do on your own.
Sorry to say, but this situation sounds bad from here.
please, let me know if there is anything else I can do to help you with this.
Hello again, hadn't heard back from you so I thought I would touch base and see if you need further assistance with this?