Welcome to CM Crossroads!!! I will start the comments. First and foremost the problem that you are having is not branch/merge related, that is simply a symptom of a larger problem that exists accrding to your description of the situation at your workplace:
1st you stated - "Project/Feature Flux:
-Projects are often delayed, rescheduled, reordered, scope is added/ejected late; sometimes in QA
-Certain release dates are etched in stone, but scope is morphable"
This is unacceptable practice and poor planning is no excuse in any development environment, I am not aware of your dev methodology, but you can't"simply" add changes in QA, fix defects and bugs but not wholesale changes to the code base. Either your in a bad Agile environemnt or have cowboy coders.
You stated that you have:
5 different teams, each doing multiple projects and each team doing many smaller tactical feature efforts
Number of developers:
40+ distributed globally; multiple sites / timezones"
This leads me to believe that you have major communication issues, ie the right hand has no idea that the left hand even exists let alone what it might be doing.
-Need to show traceability from requirements to approval to migrate to code tested to code deployed. Need to show that only the code tested and approved was migrated. [Not sure how we pass audit since extraneous code is in every release; vestiges of future projects go in too]"
Could you really have this ever given your scenario? Sounds like your audit process is weak since you later admit, some things just don't make it in, or they have to be recoded/reapplied...
Your developers are in denial because right now they are free to do as they please. Or at least thats what i gather, hence their opposition and throwing up barricades saying what you are proposing is not workable.
So in a nutshell you have much larger issues than branching and merging.