I am assuming you mean for a crown moulding installation, or other form of upper moulding.  You will need to tack wood strips to the tops of the cabinets that will allow you anough room to fit your moulding.  So, if you are standing looking at a run of cabinets, the leftmost cabinet will need a piece on the left side and from, all cabinets form there will only need a front piece until the last cabinet which will need a front and right side.  This served two purposes, one to maintain proper spacing, and two, to attach the coulding to something.  Tip:  be sure not to go through the cabinet with attaching the wood.  I usually use glue and one or two tacks (brad nails) to hold the pieces.

you would use molly bolts that will have to go thru the sheetrock, be sure to put them thru the wood stile, top & bottom. make sure that the bolts are at least an 1 1/2" longer than the wood + the s/r + the back of cabinet.

How do you mean doesn't meet the studs? Are they spaced away from them or do they not have a stud at each end of every cabinet? If they are spaced away from the studs you can use furring strips to span the distance. In most other cases if you at least have an 18" cabinet or wider you should have at least one area to screw through to a stud.

Most ceilings are not perfectly straight. You will need to install a trim to hide the gaps that will be created by the straight cabinets. Trims bend a little to conform to the ceiling.