HB monthly refunds?

I think you can't, even if you haven't revealed the keys or made gifts links.

I believe that it's possible. Just depends on how effective your message to support is. Last month where I wrote a lengthy complaint about the decreasing quality of Humble Bundle they offered me a refund, even with used keys, that they would deactivate and thus remove from accounts that use them.
I heard of people refunding even before last month's debacle.

https://support.humblebundle.com/hc/en-us/articles/217300887-Humble-Monthly-How-to-Unsubscribe-and-Refund-Policy
You cannot get a refund, although there have been exceptions. The last bundle fell on the thin line of false advertisement, with counting one short demo as a full game, so starting a dispute with them will let you get a refund on it, as long as none of the keys were revealed.

On the other hand, they still think only in American terms, so it would be an interesting thing to bring it to European courts. Here we have this:
http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32011L0083&qid=1407497640072
It states that if you buy a product on the internet, any kind of product, you have 14 calendar days after the product's delivery (so not after paying, but delivery) to say you don't want it and you want your money back. In terms of these keys, this means that you have to get your money back, and Humble just revokes each and any revealed key.
And the fun part is that this law overwrites any seller's Terms of Service, as long as you are an EU citizen, because this directive has been adopted by all members states.