Re: Carbonates are how the Earth deals with excess CO2
As long as the oxygen content remains high enough, CO2 levels pretty much doesn't matter - the lungs won't absorb it.
That's completely untrue. CO2 in the inspired gas *will* be absorbed - or at least it will prevent CO2 from the body being expelled. It's just that hypercapnia is a prefeable condition to hypoxia.
Some complain of a headache if exposed to several percent of CO2 for hours, but that may be due do other contaminates in the air
No - hypercapnia is an issue. It leads to an assortment of symptoms - the worst, IMO, being panic in hyperbaric situations. This has undoubtedly led to the deaths of quite a few divers.
Only when CO2 displaces O2 is there a real problem - but that would be true of ANYTHING.
Again - untrue. Try breathing a normoxic O2/CO2 mixture. You'll only take a couple of breaths before you get into respiratory distress. It's a horrible thing to go through.
backfill the coal mines where you extracted it to begin with.
And what form are you going to use to store it? Something soluble? That'll be nice for your water table.
 The urge to breathe in just about everyone is driven by blood pH. CO2 in your body dissolves in the blood, leading to carbonic acid, which lowers that pH. The lower the pH, the greater the urge to breathe. Carbon dioxide is an active chemical in our lives; it would be an enormous mistake to consider it an inert gas.
 There are, apparently, a (very) few chronic CO2 retainers whose ability to determine blood pH has failed. I'm fairly sceptical of this, but I've not bothered to research it in detail, as it's not going to apply in the environment in which I'm going to be involved...
 Diving :-)