Can an improper alternator Installation cause engine failure?

I bought a used 2004 Nissan pathfinder from a local dealer 2 years ago. It had just over 80,000km on it. Since I've owned it I've put on about 40,000km on it. This past winter my vehicle was having troubles starting. Chalking it up due to typical Canadian winter I started plugging it in and would find it still dead and only a boost would resurrect my pathfinder. Then I noticed dimming headlights and the battery wouldn't hold a charge so I managed to get it to my family's mechanic but it made a horrible high pitched squealing noise the entire 20km into town. The mechanic told me it sounded like belts. He could hear my vehicle from inside the building. I left it in his care and went to work where a couple hours later he calls to tell me it's worse than he though. It was the alternator. He replaced the belts but upon starting it to pull it out of the shop the squealing was still there. I gave him the go ahead to replace the alternator and he threw in the belts and their labor for his error. I get my truck back and it's working! No issues starting up and no more squealing! Then what my family called the haunted truck started. I'd go to my vehicle in the morning and the windows would be wide open. This happened numerous times after the alternator issue. I'd also find my interior lights on including those little round reading ones or whatever. Then in the middle of winter out of nowhere the floor vents were blasting cold air and the windshield vents were blowing hot air and both the dials for adjusting the amount of vent airflow and the one that controls which vents are in use froze up and wouldn't turn and if you'd try they'd click and be hard to turn. I took my truck back to the mechanic. He looked at it and said he couldn't find an issue for the haunted truck and I could take it in to the dealership but they'd charge me a bundle. A single mom of two with a new job and finally just bought a house I was in no position to do that especially after a hefty $900+ bill after the alternator so I ignored the freezing feet as much as possible and the haunting seemed to go away. For a while. The weather warmed up and suddenly the vents and heating and even the dials worked perfectly fine. I thought I had finally got some good luck! The window was open a couple morning still and that pesky interior light too but my mechanic said it was fine. 2 days ago I turned a corner and a loud embarrassing knocking started coming from the engine. Driving the kids to school I tried to pretend nothing was amiss although I made everyone's head turn with the racket. I checked under the hood the coolant was a little low so I filled it with water just to get me to the auto shop checked the oil and it was fine too other than it couldve used an oil change at anytime which I knew be ause my kms were close to my sticker. I drove from there to the same mechanic as before and he hardly hesitated before saying the engine was failing and I would need a new engine. He said he'd check to make sure and call me at work. I had to track him down to get a positive on the engine failure then he proceeds to tell me he doesn't know if he can do it. He only has one guy helping him and an engine replacement is too much work. I'm female. My knowledge in this area is extremely limited if not nearly nonexistent, however, is it normal for a 2004 Nissan Pathfinders engine with less than 130,000km would just completely fail out of nowhere? I owned a Dodge Neon that cost me 1/10th this vehicle did and I drove that to almost 300,000km before it died. This doesn't make any sense to me. I love this truck. Took care of it better than anything else I've owned and I'm totally devastated and have no way of finding out what happened. Please help. I'm sorry if I sound long winded here but I didn't want to miss a single detail.