At this point, if you haven't already, make sure you are putting all of this in writing to UA. Fill out all of the forms required, send e-mails, etc. You may need this if you go through your credit card, and putting things in writing always seems to work better with airlines. Calling is ok to check status, but document claims and info in writing.
FWIW, I'm not sure getting into an ugly row with the GA would have helped - you still may have wound up gate checking. With hindsight, perhaps stepping aside and telling the GA's you needed a couple of minutes to rearrange bag contents would have mitigated some of the damage. Done politely (and getting out of other passengers' way) will buy you some extra time. If you have memory cards or similar perhaps you could have pulled those.
One final thought - have you gone back to EWR UA baggage claim to see if your bags happen to be "misplaced" there? Every now and then bags end up stranded in some remote corner of the UA baggage claim area. This worked once for a parent of mine. Don't get your hopes up, but you said you were desparate.