13' STi died while braking to a stop sign

[12GA]

I bought my STi last year in July, it had 129,xxx miles at the time of purchase and ran fine. After a couple of months it started to have a light grey smoke come from the exhaust on start ups or when getting to a higher speed on the on-ramp. I decided to do a combustion test to see if I had a head-gasket problem but it came back good. I then tested compression and had about 120PSI on each cylinder. I thought maybe it was just something that would go away (I know that's dumb to assume but I was busy at the time and just needed a car) but after a week or two I ended up having a miss-fire in cylinder 4, I instantly replaced all 4 spark plugs and coil packs which ended up with the misfire code not showing up anymore. A couple of weeks after that my check engine light showed up again with a code for the CAT. I set-up a appointment with a mechanic to have it looked at and a week before I was scheduled to go in I was driving home and my car died while coming to a stop. It was so quiet that I didn't even realize it until I put it in gear, I tried starting it again but nothing happened except for it trying to turn but no start. I tried to diagnose it by having my battery and alternator tested which the battery came back good but the alternator came back as failing so I bought a new one but it still didn't wanna start. I tried checking the codes again and kept getting random codes related to my ECU. I took it to a mechanic and he refused to do a leak-down test or anything else and said just by hearing what I told him that it was the engine and it would need fully replaced. Do you guys agree with his diagnosis or is there something else I should check out before going the route he offered?

 

[Ecmbuster]

A "mechanic" with a slight amount of knowledge would use a long ratchet or power bar on the crankshaft bolt.
Rotate the engine clockwise, if it does not turn, there is a mechanical problem.
"Random codes" I suppose the forum readers should guess?
Pressure test the cooling system while cold. With a "boroscope" signs of a leak can be found.
Pressure test with a pressure transducer at the cooling reservoir at idle. Pressure spikes and a "trigger" on oscilloscope, will measure a possible leak to the cylinder that may have a leak.
Pressure test the oil pump at the oil pressure switch.

 

[CocoTaco]

For the love of god find a new mechanic if he isnt even willing to do what i consider basic testing to condemn an engine (compression test/leak down) or even baring it over with a ratchet as ecmbuster said