Sagging cathedral roof and adding new insulation? Need help soon

I need immediate advise. I have house ( ranch) built 1950`s, it has 2x6 cathedral ceilings through out the house. Eleven years ago I had re roofed the house and not checking out what a hot roof was, I put ridge vent on and opened all sofitts. Thinking roof should breathe. Now we had hail storm and roof was damaged. Since its poorly insulated ( fiberglass batts) I decided this was opportunity to add closed cell foam and using osb sheathing. Insulating contractor says to spray 4" closed cell foam leaving 1-1/2" ventilating channel. I also have roof sagging over 20x15 living room and 15 x 12 bedroom. After all my research I could have rafter rot and mold. Will need to put new rafters next to the old when we open from top side. I consulted structural engineer that was willing to talk on phone without a charge ( could still hire). He did not recommend vented roof with closed cell foam, he said the foam will shrink and I will have future moisture problems. He said I need some type of moisture barrier against sheet rock. He said put hot roof back on. Insulator says foam will not shrink. Who is right? He said we could do 2" foam and spray fiberglass to fill whole cavity and go back to hot roof design. Do not put ridge vent in and close off soffit. My research say for hot roof design must fill entire roof with closed cell insulation. Does anyone have an opinion on what should be done. I have read so many articles I am now very confused. The one article I was following
http://inspectapedia.com/insulation/Insulate_Cathedral_Ceilings.php shows
retrofit correction of cathedral ceiling. New roof scheduled to be put on in about a month.

Mike

I have had 36 viewers and no one has an opinion about any of my problems?