Can one do a giveaway for 'GameMaker: Studio Pro' with a yoyogames key as it gives a steam key?

Wasn't that the key you had to wait two weeks for your Steam key first? I'd say best not to, simply because they can't confirm it for two weeks.

Wait 2 week, then get keys from then and then mage gibs.

I'm going to say no, because they only have a week to mark received and they can't mark it for two weeks.

I think it will be linked to your YoYo account since it is generated on your account.
This could lead to unexpected and unwanted behavior in the future.

There is a lot of info in this thread

Can one do a giveaway for 'GameMaker: Studio Pro' with a yoyogames key as it gives a steam key?

No, you can't. You need to create a new Yoyo Games account, redeem that key, wait the 2-weeks to recover the Steam key, and then make a giveaway. Making a giveaway with a non-Steam key will get you suspended.

I'm not sure if this things against Yoyo's ToS or not, hope you didn't get a trouble with your Yoyo account.