I have done somewhere between 2,-3,000 dives in my lifetime. On two occasions I experiencing situations like what you are considering above.
The first was with rental gear in Thailand (Koh Phi Phi). We were descending to the 100 ft level at Chumphon Pinnacle when something blew in my first stage. First thing I did was check the SPG to see how quickly air was escaping. I had plenty of time. Second thing was to tap the shoulder of the boat's DM ahead of me. This was at the start of the dive so we hadn't been down for more than 2-4 minutes. We ascended and swapped out regs and tanks on the boat and continued to dive. I never got the "autopsy" on the failed first stage. It did fail wide open (as they are designed to do), so I had sufficient air to recover.
The second was with my own gear here in Catalina's dive park. Although I almost always dive with my pony at home, I didn't clamp it to my tank since I only planned to go to 40 ft (an easy ascent for me). About 3 min into the dive I had chased a fish down to 70 ft while filming it on my descent. I exhaled and tried to inhale from the reg. Nothing, nada, zilch. I got no air. As I grabbed for my octo, I began rising towards the surface. Nothing from the octo so I checked the SPG. It read 0 psi on a tank that I had just checked at the surface where it had 3,200 psi. I'm certainly no "hoover."
I continued to do the emergency ascent from 70 ft slowly. I didn't panic, didn't drop my weights and made it to the surface in about 70 sec. Textbook emergency ascent. Since I had only been at depth a minute or two, I wasn't woried about getting bent, just embolizing.
Both local LDS'es inspected my tank and reg. There was no sign of a problem with either. The only thing we could figure was that while descending head-first, my debris tube clogged stopping all air flow to the first stage. Now I descend horizontally or feet first... and always carry my pony (well, except for the 133 ft solo dive I did at Ship Rock Sunday. I just HAD to risk life and limb to film a subject down there.) My pony went out of hydro recently (I forgot it was purchased a year after its initial hydro) so I couldn't fill it. I'm buying a new one (19 cu ft) day after tomorrow and will have the 13 cu ft. one hydro'ed as a second backup.