The crown placement would not likely cause the adjacent tooth to have problems.If you had a large existing filling on the tooth, it is possible there is just a coincidence. Fillings act like a 'wedge' inside of a tooth and can lead to fracture. If it was a 'perfect' tooth with no filling,...
You can get a complete metal free crown, made out of zirconium and it will be strong and provably depending on how much surface is left, it would not need a root canal, this is more intended to be done, if there is not too much support tissue left. but with crowns aesthetically they will...
I'm a little confused by your question. Is the dentist saying that there is a cavity in this tooth?If nothing is 'wrong' with it, I'd leave it alone! If something needs to be done due to a cavity, I agree with the prior answer in that a composite filling with a fiber-post would suffice to...