I wish you and other many fearful flyers read my past FAQ as all those that you have just asked me have been covered before. I received so many questions daily that it is not possible for me to reply to each and everyone anymore - except when I am free ...and then I will entertain one or two. (Even though I have upgraded and transferred most of my old posts to this new site, I regret to say that I have not been able to fix all the broken links - there are about 1000 FAQ!)
Tonight, you are lucky... I will repeat or point out some links to your questions.
For a plane to crash during the cruise - it is extremely slim but it can happen. See here. Even if a plane has lost all its engines, it is still flyable - see this video “Flying on Empty”.
Do not worry about any diversionary airports along the entire route in the event of an emergency. Pilots plan their routes carefully for the flight would be illegal if no alternate airports were nominated. See one FAQ pertaining to the Boeing 777 on a route between Perth and Dubai here. Once you are over the land, there many airports to choose from.
You can read the topic on Ditching here regarding planes crashing onto water. If a ditching is well executed, it is likely that the plane may stay afloat initially if it was intact. So there is no question about the doors not being able to open.
On a prepared ditching, all passengers would have been briefed to wear their lifejackets prior to leaving the plane. If a plane is submerged, then there is a chance that it may not be in one piece. So one has to escape through any cracks rather than trying to open any doors!
A plane will not nose dive unless it suffers from control problems. It has happened to older planes in the past but modern planes today have many redundant controls to make such a thing happening very remote. In fact, a plane without control problems is inherently very stable during a dive. It will recover when it regains its speed - something like a paper plane!
For the explanation about turbulence, there are many FAQ here.
Meanwhile, you can watch some videos on Fear of Flying as explained by a retired airline pilot (Captain Stacey) below:-
If you like what you read, more stories are found in my book LIFE IN THE SKIES (Preview here) and you can purchase a copy here. To check for any latest updates or postings, you can follow my Twitter at @CaptKHLim or Facebook here