What, if anything, can save this car from the scrapyard?
Weirdly, the thing that bugs me is not that it might be curtains for the car but that I don't know what the problem is.
What can save this car from the scrapyard is
you admitting that you don't know what you're doing and leaving it for 2 hours to a reputable independent mechanic (dealerships mostly deal with new cars), pay the man a hundred bucks for some exotic sensor that looks like a butt plug, throw in a couple more for his cool beverage of choice, and drive away happily. Most of the threads I read on engine problems (and I read all of them here and on
mbworld.ord) end up being either MAF or some other sensor. I don't recall a spark plug or a coil sorting itself out after warm up.
The thing that bugs me is that you consider taking it to a garage "risky", but you are perfectly fine spending a crap load of money beating the problem by replacing parts that are perfectly fine to begin with. At the same time - you said
your head gasket went, but didn't mention anything about replacing it. You said
your crank case cover is cracked, yet no word on getting that sorted. You top up half a litre of coolant regularly, and so no and so forth... You see the pattern?
And please don't feel like I'm attacking you. I think you've invested way too much in this already and don't want to just suck it up now. I've done this before and I know I needed a good slap in the face to stop being a moron.
Tackle problems one by one, starting with the known issues and eventually the culprit will reveal itself.