Accidentally clicked "key" on Humble Bundle instead of giftlink--is giveaway invalid?

You can use the key as long as you don't redeem it on Steam. The only problem you may have is that if you repeatedly reveal the keys and not redeem them on your personal Steam account, they're going to label you as a "Key Reseller" which results in no support from Humble Bundle regarding faulty keys or creation of new gift links.

So yeah, don't worry about it :)

yes