So general note: Almost EVERY movie made has elements of luck and happenstance to drive the plot forward. Books are the same way.
Now, to 4 and 5.
4) Han Solo clearly states that he was searching for the Falcon and was tracking it's signature. As soon as the ship was started up by Rey and Finn, Han Solo knew were to find it. Not happenstance, and it was explained.
5) You mean he knew that Captain Fasma, the ONLY shiny silver storm trooper captain in the movie, would know key strategic information like shields? Yeah, I'd say that's believable.
I'd like to know why the storm troopers are such terrible shots and leave themselves exposed to kill shots all the time in battle scenes. You'd think the empire would train them how to engage in combat a little better, since they're trying to destroy the rebels.
I know it's a movie but the troopers are like 0 for a 1,000 on shots towards the good guys.