I hope for your sake that nobody does try and get off in Glasgow if they've been checked through to Manchester! I can't help with suggestions as to who you could take this up with and this probably isn't the best forum for checking out the information you need because as yet you don't really have a complaint - the airline is still providing you with exactly what you booked, that is a return flight from Manchester to Las Vegas and back. I would suggest that you post your query in the Airlines/Flight only thread as a number of people who have access to information that isn't generally available about aircraft movements are regulars there.
I would think that they would need the following information:
Tour Operator/ Travel Agent (if any) that you booked with,
Whether it's part of a package or you booked direct,
the name of the airline,
Whether it's a Charter or Scheduled service (if you know),
Flight times and flight number for both your original booking and the amended details if now different,
Dates of both inbound and outbound flights,
Arrival and departure times for both your original flights and the amended flights if different.
If they were consolidating flights I would have expected the outbound flight to have also embarked passengers at both Manchester and Glasgow if the return flights are doing that but this appears not to be the case. One question to ask of the airline is 'Where did the people who will be disembarking in Glasgow on the return flight start their journey from?' And it's important to note that sometimes 'open jaw' tickets with scheduled airlines (ie returning to a different airport than the one you left from) are possible but depending on the airline can cost you a lot more than a straightforward return. Did they say that you couldn't return to Glasgow full stop or that you couldn't do it on the type of ticket that you had without re-booking at extra cost?
SM
PS Having just seen your more recent replies I think that the fact that it is a charter rather than a scheduled flight could be significant.