Plz Help? Which 1st? Loud squeal,stuck in park when cold, service engine soon

What is your question?

Having a few problems that started and need advice! What to deal with first & where to go? 1. For about a week or 2, I can't shift out of park for a minute or 2 when it's cold & has sat for a few hours, then it's fine. (Trans rebuilt at an exactly a yr ago & still under warranty). 2.Coming home on freeway, my "service engine soon" light came on & stayed on. 3.Next morning, I started it & to loud noise. Best description is a rough sounding consistent squeal....similar to noise when my serpentine belt went.(Went to nearest oil change place & they replaced it along with the tensioner 6 mths ago). I checked the belt & did notice about a 1" area on the edge of belt that was kind of white, but other than that, it looked fine. Also looked at it when running & as far as I can tell, looks fine but I'm no mechanic. Took back to them to check it & was told that belt & tensioner still fine & he thinks its a bearing in the alternator now?? (I've read that incorrect alignment/overtightening of belt could actually cause the bearing to go??) Thoughts on that? (No other symptoms ..no lights dimming, probs starting, etc. ( recently noticed when on freeway, occasionally int lights come on for a few seconds but my drivers side door hinge is broken/door doesn't close tight so have about 1/2" gap around the top of door - assumed lights due to vibration at high speed & still not sure...I've been climbing in from passenger side & hoping to get this fixed b4 it gets any colder but quote that I got was too high considering it's $20 for hinge & about 1/2 job...Now bigger things to worry about. Heated seat hasn't worked for a long time either & no one has been able to figure out why. They ran OBDII for "Service Engine Light" & said Cylinder 6 fault...(didn't tell me actual code looked it up - found P0306 & read that engine hesitation can be a symptom –( I have noticed this occasionally for a while now. (hesitates when taking off fast from dead stop, then will suddenly pick up speed like it should). As a woman, I dread walking into a repair shop & just want to be armed with as much knowledge as I can before I do so I don't keep throwing good $ after bad.. Need advice on type of shop to go to & aprox cost to have cynlinder 6 & alternator checked (also, what should be included? Can an auto parts store check anything for me first? (The engine light is no longer on after he ran the code).I don't want to end up replacing the alternator only to have it go again because there was really another problem. Or - should I take to trans shop 1st to see why probs shifting out of park when cold since it's under warranty? (Guessing trans shop not the best place to check the new issues though). Driving a borrowed vehicle for now so need to take care of this quick! Thanks in advance