I'd be quite dangerous and very uncomfortable: exhaled air contains ~4% CO₂, 16% O₂ and is at roughly 100% humidity.

* at 4% CO₂ you'll be suffering from moderate CO₂ toxicity (narcosis, dizzy spells, increased heart rate & blood pressure, mild shortness of breath, exaggerated response to effort)

* at only 16% O₂ (but assuming atmospheric pressure), you're well into chronic hypoxia land, not quite sure lethal but not far, the hyperventilation from CO₂ poisoning will make this slightly better but nowhere near good

* the combination of hypoxia and CO₂ poisoning will make effort (let alone exercise) possibly to probably lethal

* the extreme levels of humidity will make the entire thing even worse as you'll have a hard time cooling down

You can probably live in this situation for a few days, but the CO₂ levels alone would make it lethal past a few weeks, the combination of all factors means likely no more than a pair thereof, at best.

Well that's how CPR works ... I can't remember the details ... it's many many years ago I studied first aid but I think it's something like 1/3 of the Oxegen gets used, so when you're giving the kiss of life you're imparting the remaining 2/3

Can't answer the question, but I can say that when I did HUET we'd breathe into a bag in the suit and back again. This worked surprisingly well for the time we needed to stay underwater before we could escape. I'd still warn against implementing ones own version of this without studying the topic well, there's a number of pitfalls that one might not be obvious of up front.

I've briefly considered if there might be something to remove CO2 inside the bag, but decided against it since this was training equipment so not sealed and probably reused multiple times.

CO2 concentrations above 7% are enough to suffocate you even if there is otherwise ample oxygen.