Sorry for the basic question, but my corporate travel agent is giving me info that seems to be in their self interest, and I need to get an answer before I start traveling in 3 or 4 days.

I have already ticketed a DONE through my travel agent on Cathay Pacific ticket stock (my first segment is on CX). I would like to add a few more segments in North America but my travel agent states that they have to cancel the old reservation and create a new one for any such changes. Is this correct? Of course, my travel agent collects more fees everytime I do this. Because of this hassle, my internal travel booker wants to issue the segments as a separate ticket which is a waste of a DONE in my opinion.

Anyway, if I have already started travel and need to make changes to the segments, could I bypass my corporate travel agent and call one of the airlines directly to add additional segments to my DONE? To be specific, I would like to add two domestic AA segments as my second to last segments (just to reconfirm, may I stop over in the same city more than once?). These segments would cost a lot more than the standard USD125 change fee charged by Oneworld so I am keen to not have to buy separate tickets.

Last question (again sorry it is so basic), if I have a DONE and there is no D (or A) inventory, may I book any available economy class seat or must L inventory be available?

Thanks for your help!