Thank you for taking the time to respond to my questions.
The information you have provided me gives me a much clearer understanding of what the situation is and has allowed me to suggest a diagnostic approach.
I am sorry to tell you that you should NOT spend any more money on this engine if it has been overheated. If the oil and antifreeze has mixed you have a serious problem. It has been my experience that this 2.7 litre engine does not survive overheats. I am very surprised your mechanic advised you to put a water pump in before he figured out why it wouldn't start. That's very very bad judgement, in my opinion. Did he replace the starter at the same time ?
Don't let anyone "throw" parts at this car. That's a ripoff. A lot of people used to get away with that, but your car is too complex (and it's got too many expensive parts) to deal with this that way.
You need to find a reputable shop to get this tested. Alternately, you need to find someone that knows how to use a voltmeter and get him involved. I can instruct him on how to do some tests to see if we can figure out why this won't start (without replacing a bunch of parts). If you don't know a good shop, tell me what city you are in and I can refer you to a contact who can refer you to a local shop.
How many miles on this car? You may need an engine replacement. If that is the case, you need to check the resale value on the car first. If you were to invest big dollars into this vehicle and it was involved in a collision several weeks after the repair, the insurance company settlement might not cover your investment. Think about this carefully before proceeding , if it comes to that.
So sorry to be the bearer of bad news. Let's not assume the worst yet, though. Let's find out why it won't start.
Please decide which way you want to proceed and get back to me.