Cannondale Trail SL 3 component upgrades
My understanding is that 5 speed is unique, 6/7/8 all share the same chain specs (unless its weird) and that 9 and 10 and 11 are all unique again.

You're probably best to keep everything in the 9 speed range, and perhaps enjoy some tech trickledown from the higher grades.  

Your specific questions


A 10 speed deraileryeryer will probably work "okay" but the wire pull is different, so in the middle of the cassette should be fine, but as you approach the outsides it will progressively not go quite far enough.
You can use any 9 speed shimano shifter on a shimano 9 speed setup.
The crankset does not need the same number of teeth, but if you make it too big or too small, then the front mech may be too small to span all the chainrings.


2013 is not that long ago - for a bike part to be worn out in a couple years suggests that you're using it well, or mistreating it.   Care for your ride and it should last 10-20 years.