We had a similar experience on our 2010 purchased in early 2015. Car was driving normally, began to shudder and quit at 5 mph on a busy street. I personally put Royal Purple oil and filter in it, but engine would not turn over. The engine had dropped a valve, which had broken and the stem went into the intake along with fragments. Not finding a used engine, I went with new GM 3.6 and after independent mechanic fiddled with it for a year, duriing which it moved less than 50 feet, it was towed to a GM recommended dealer. They reworked the heads of the new engine because the valves were damaged by reusing the plastic manifold with fragments in it. I paid. Then engine was assembled and it ran terrible. There was metal in the oil of the new engine. Another new engine was installed. It ran, but was sluggish. A few weeks later, it had insufficient power to climb a hill to go to church, had to get a ride home. Catalysts were plugged as was muffler. All were replaced with new GM and it ran better, not as hoped. I Use Mobil 1 oil, tried Cataclean a few years later and it seemed to run much better. I recently drove from coastal Alabama to Virginia, through other states and back with no engine trouble, though gas mileage averaged only 23, less than the big LT1 powered Roadmaster wagon I traded.2015 buick enclave all service and oil changes done per schedule. No problems at all with the vehicle until the other day. Drove 10 miles on interstate with no issues - no warning lights, messages, smells, sounds, vibrations, and temperature reading was normal. After 15 minutes got back in the car and left the parking lot, as I pulled out onto the highway the car began to stutter like it wasn't getting enough fuel. I noticed the service stabillitrack message followed by a traction control message, then a reduced power mode message and then engine light. I pulled of to the side of the road and came to a stop. Then the engine stalled. I waited a few minutes and tried to start the engine but it would not start and barely turn over. Then I got a battery low, start engine message. Checked the oil and it was fine. There was no smells or fluids leaking. This is a well maintained 2015 enclave with 130,000 miles with no previous issues. The battery is the original but never had an issue with it. Had car towed to dealership and was informed the next day that the engine had seized up. The evaluation from the mechanic stated that the starter only clicked when he attempted to start. And that he was unable to turn the crankshaft balancer manually due to major internal issues. Recommended engine replacement at a cost of $9,000. Fault codes were P0068, P0300, P0101, P0106, and P0513. This is the evaluation I received at a cost of $146.00. Although the vehicle is in great shape inside and out, we are reluctant to spend $9,000 for the engine replacement. And are leaning toward selling "as is" and looking for a more reliable manufacturer. My question is whether it would be prudent to pay for additional evaluation. The mechanic stated that he is unable to determine the actual failure point without removal of oil pan and cylinder head. Is it worth paying for further evaluation?
If I had to do it again, I would just install a good used engine and trade for a newer car with a warranty. I spent a year and over $15K to fix this car. I have a 2010 with a very low mileage engine now, have done brakes, shocks, and struts, so it drives like new.
So sorry for your trouble, be prepared for bad news.