I, personally, wouldn't use the "Suffocating" rules EXACTLY as written. This isn't some form of gas or choke - this is more internalized damage, to one / both lungs (sounds rather serious). You can't really hold air in a collapsed lung so the "hold breath" time should be halved (i.e. one collapsed lung). I would also probably add some kind of disadvantage to the Tarrasque.

Also a +1 from me for the suggestion to "play dumb". Remember, as a DM, your role is not to defeat the heroes at all costs; your role is to be the architect of an epic (or sad, or tragic, or scary, etc.) story. If you're playing a heroic RPG with powerful heroes, then the DM should strive to keep that overarching tone of the game. (Obviously that doesn't mean heroes can't ever get their behinds smacked - just that the overarching tone should be preserved!)

Thieves can't what? Ooooohhhh....

If the context of using suggestion was different, I'd allow it under certain conditions - primarily that the effect might not be exactly as advertised. I.e. think about dreaming up a cool idea, but then you wake up and it gets all muddled. If this had the potential for some comedy, why not? In the serious context the OP posted it tho it seems like a bit much.