Answer to the last one is an easy "yes" by rule -- dropping the first segment, so long as it doesn't change the award (it doesn't) is definitely no problem. See here:
https://www.flyertalk.com/forum/amer...ancel-etc.html
As for swapping LAS for SAN, I really do think you can call and just change that without risking losing the other segments, but just be clear with the agent that you want to make the change only if you can do so without affecting the rest of the flights. If you get someone who doesn't understand, call back. As EXP there are no fees for any of this.