Exporting Steam keys from Humble in bulk?

Also, some keys - especially older ones - will have already been used. Is there a way of verifying what is an unused key?

no and no

you got to copy/paste the links individually, and you have to activate a key to find out if it's been used or not

maybe you can contact support. but i doubt that

Oh well, thanks any way!

You can copy paste them to Excel and then use some simple formulas to get them in columns. That's how I do it.