"Would you be better dumping your whole regulator and breathing off the bottle by cracking the valve as necessary?"
Breathing directly from a bottle is more stunt than practical.
If you’re diving a single with single valve, and within NDLs, a controled ascent and switching to your buddy’s secondary as you empty to complete a stop if necisary be a far better strategy. If your buddy misses the "show", time for a new one.
Had a free-flowing reg years ago, was able to "feather" the valve to slow the flow so I could breath the reg with less waste, free-flow stopped as I went below about 1800 psig (from a 3500 fill). Actually shortened but did not abort that dive. But you’d have to be calm and thinking (and able to reach your valve), not anxious, to do this.
These days, would just close the appropriate valve on the dubs and turn.