2x8 foor joists above not supporting the roof
no, you really don't supply enough info on the weight loads this will hold but you may need to add a steel plate between them, also you need to survey the area where the posts rest and determine if this is strong enough for the transfer of the weight load you are taking a dispersed load and turning it into a point load, I have seen large headers put in and the floor where the posts rest start to sag because that area wasn't strong enough to support the point load
Nope! You'll have to use a double 1 3/4 x 14 LVL. The best you can do with a double 2 x 12 is between 6' 3 and 8' 1 depending on the width of the building parallel to the joists.
do you want to make a header for a 14 foot cased opening -- 2 boards of two inch by 12 inch wood are sufficient and you do not need any plywood in the center of the header ... if you are planning to panel and are adding a room type space then standing