Second question: is an airways/CAS rejoin guaranteed after a section of flight exits CAS for GA aircraft?
Officially, no, not guaranteed.
Practically, quite a few situations where it is as good as guaranteed but if in doubt just check. If you have filed or are limited to a level and the airway base steps up above you before coming back down, then it would be very unusual and more than a bit harsh to not get a clearance back in! Some examples, p600 ADN to PTH at FL 100 or below, N601 POL - TLA at FL120 or below in daytime.
Hope this is more use!