1) When fully collapsed, it is a pipe inside a pipe. When fully extended, it's no longer really a pipe within a pipe. Therefore, it'll be approximately double stiffness or single stiffness, depending on which configuration the slip joint sees. To that end, there's not really a good way to demonstrate a variable stiffness of an expansion joint in CAESAR, in this manner. So, pick one that's conservative.

2) We use the (corrugated) expansion joint in caesar because both will see pressure thrust, and lateral stiffness.

When a corrugated expansion joint is compressed externally (let's say, by an operator driving a forklift blindfolded), it will resist the load in increasing amounts due to its axial stiffness, and then, if the external load is removed, the corrugated expansion joint will push back to its uncompressed form.

For a slip joint, while friction will resist the forklift, the slip joint will not go back to its original shape when you release it.

When we say zero stiffness, that does mean if we calculated forces for the forklift scenario, that zero transmission at the slip joint anchor will be reported by CAESAR. This is, obviously, incorrect, but also beyond the scope of CAESAR. I would suggest looking at the loads on adjacent supports if you're required to design the slip joint anchor.