I agree with you that if the camera is worn but isn't on during an incident that the onus should shift at least somewhat towards the officer - I just don't believe this is what actually happens.
When it comes down to the nuts, I think most would still most often side with the 'most likely' thing to have happened - that the cop just forgot to turn the body cam on. It is an improbable leap, imo, to say that the cop turned it off or failed to turn it on on purpose without any supporting evidence for this position.
Your reasons you list for not having cameras recording while an officer is on duty are mostly valid - though some could easily be regulated by procedure.
For your first bullet point, my example of a 'suspend recording' option (that can be audited) solves it.
For your second bullet point, there are many occupations whose every movement (and sometimes words) are recorded the entire time they are on the job - bank tellers, fast food workers, tech support people, etc.... there are many others - and maybe they all don't wear body cameras, but the levels of workplace surveillance can be even greater using the tools that are already in use. Plus - and this is important to my thesis: these positions aren't entirely reliant on the public trust to be effective - like law enforcement is.
"Nobody should feel like every word and gesture is being watched and reviewed."
I would maintain that nothing is being 'watched and reviewed' unless conflicting accounts of what went down occur. Is being recorded unsettling? Absolutely. But it certainly is not unique to the law enforcement occupation.
Bullet point 3 I agree with you on informants, but if I'm a cop I want the witnesses statement to be recorded on site. I can see where some might be reluctant to say certain things if they know they are being recorded - as you allude to.
My position regarding recording everything while on duty clearly should have some built-in exceptions - like some of the ones you mention. But to rebuild trust in law enforcement (which I believe at least a part of this initiative is attempting to address) the general public must believe in the transparency of the process.