Accidentally killed my mussels

Question

I bought some live mussels from the fish counter and I had been reading about 'flushing' them so I put them in a bowl of fresh water for a while and carried on perusing the internet.

I then read here and here that leaving them in freshwater will quickly kill them. So I quickly rescued them and right enough a few of them are now dead (slightly opened and won't close no matter how many times I yell 'clear' and tap them on the worktop). Lesson learned.

Before I soaked them I checked they were all alive - they were all either closed or closed themselves after tapping. So any that died were killed by me slightly earlier than planned. The rest are now cleaned, de-bearded and chilling in the fridge mourning their fallen comrades.

Although in the comments for this answer they seem to say to use fresh water to tell if any mussels are dead rather than saying it will kill them. But anyway...

Problem is that now I have a load of Schrödinger's mussels that could be alive or dead as I might have killed them. I've found LOTS of debates online about whether or not you should eat a cooked mussel if it's still closed (consensus and most research - one example - says yes it's fine). What I can't find an answer for is...

1. Are dead mussels always open? Everywhere says if it closes then it's fine, but is it possible to have a closed dead mussel?
2. Are any mussels that died in my 'care' still ok to eat as I know they've not been dead and rotting for ages and are still relatively fresh?
3. Will dead (recently deceased) mussels still open when cooked?

UPDATE:

Thanks @logophobe for the advice :)

Answer 1

In order:

1. No. It's possible, though not especially common, for a dead mussel to be stuck closed; they'll likely open with very little resistance.
2. Most likely yes. The main issue with eating dead mussels is that it's an indication they've been poorly cared for; you can't know how long they've been dead or how fresh they are. In your case, you know with reasonable certainty that they were alive until very recently so they're still fairly fresh.
3. No. Mussels open when cooked in order to "breathe"; the excessive heat overwhelms them and they're trying to vent a little bit (which cruelly just seals their fate, but we as cooks benefit). If they're dead, they won't have this reflex and they won't open on their own. As alluded to above, most of them will be partially open already.

I would still cook them, but do a couple things to be as safe as possible:

1. Cook them without further delay, within a few hours if possible. Any dead mussels will start to go south faster than live, since they're no longer respirating.
2. Discard any mussels that don't open after cooking. Chances are these were among the small contingent who stayed closed after death and you don't know exactly how long they've been toast. Better to play it safe and accept the loss; besides which, it's a bit of a chore getting them opened up when you've got plenty of their conveniently open brethren ready to eat.