About making giveaways of Humble Bundle Keys.

Humble provide the key, only the dev can see who activates ... so as long as you gift the KEY and not a Humble Bundle gift LINK you are fine.

Are the keys revealed already? If not then once you click reveal you are given fresh key, brand key off the rack, no issues. If the key has been revealed ... well ... different story.

Humble can only tracks gift links but can't track keys, so don't use gift links, as long as you reveal the keys to gift you are fine.

For keys that you aren't sure, I mostly use MJT, sometimes DGAIKINW or Lootboy (people will automatically allow you to delete GA that you can't provide due to reasons), or even here.

If you don't care about CVs, just come here or here.

After joining those groups, make sure you comeback to SG and click on the Account button --> Sync Account so those groups can appear in your group list choices.

A) Don't generate gift links, only give away the keys and you will be fine close to 100%

B) There is no longer a guarantee that old keys are still valid. I would advise against doing normal giveaways and either give them away via groups specializing on this - for example, this one - or via whitelist where you know and trust your winners or via this forum, for example this thread. If you choose the last option, better obfuscate the keys a little bit, otherwise bots will snatch them up.

As others have said:
Use keys for giveaways not humble links.

2 month old keys aren't that old and its uncommon that keys get invalidated. If you happen to have one that is a duplicate activation... Most of the people on this site are nice and let you delete the giveaway. Even in public giveaways so you don't have to join a group..

If you want to avoid the majority of bots/auto joiners then I'd recommend level 4 giveaways. That takes care of the majority.

Other than that: I wish you well with the giveaways, I know how time consuming it can be. :)

I'm almost in the same situation, but with less keys available. Also they are unrevealed yet.
I took the oldest one (from 2012) and it was successfully activated on my account.
Now I plan to give some of the remaining keys within a Delete giveaway if key is not working
If those will also be activated without any issues - I will giveaway the rest more freely outside this group.

As othes have said, what I always do it's just to reveal the keys. So Humble can't know if I'm storing the keys, or giving them away, or activating them myself.

Just wanted to add that since you are a European citizen, EU consumer laws protect you from Humble's BS terms of service.
If they ever made trouble, the magic words are "EU consumer laws" and they usually back off.
Their ridiculous witch hunt for "traders" isn't worth the hassle and potential cost of litigation.

This being said, it bears repeating, as long as you make GAs using the keys and not gift links, there's no real risk of even running into problem with support.

The first point has pretty much been beaten into the ground now, so I won't bother... but I do want to add to the second point.

Humble has no way of knowing whether a key has been used or not, which means they don't have the ability to revoke "unused keys". If anybody is revoking keys for never being used, it's the devs. They're literally the only people on the planet that even knows whether the key was used or not to be able to make that decision. As such, a revoked key would 1: be out of Humble's control, and 2: be on a case-by-case basis.

Older keys do have a tendency to stop working eventually, but that could be for several reasons that have nothing to do with "unused", and would also be out of Humble's control. An example would be if the dev modified the game in some way, and instead of updating the game the key is for, they deleted the old one and uploaded the new one, in effect creating a completely different game (as far as Steam is concerned) with its own set of codes/keys. I've seen that happen a couple times when a game went from purchased to free-to-play, for example, but then again if it's free, you don't really need the key anymore.

And, of course, things like GOTY versions are considered completely different versions of the game, so (just as an example) original "plain" Borderlands 2, Borderlands 2 GOTY, and Borderlands 2 Enhanced are 3 different games, each with their own sets of keys. If they removed the "plain" Borderlands 2 game from the store, it could (not guaranteed, but it's possible, depending on how it's done) make any "plain" Borderlands 2 keys stop working. Another example is the X series (a space sim). I bought the "X Super Box" when it was on sale. I still have my "X Super Box extra content" that I can download at will ... but now nobody can get the X Super Box because they deleted it in favor of the Anniversary Edition (which has everything the X Super Box had but also added the game/DLC that came out after the X Super Box).