Yeah, the entire Voyager crew armed themselves with them.
Thanos is smart enough to get his hands on one of them.
Or better yet, convince Q to make him another Q, and then deal with him. Getting omnipotent power is what Thanos does, and Q isn't exactly the smartest omnipotent character around, although he's probably a step above most Marvel abstracts, and way above Mxy..
Closed. Thanos without an almost all powerful artifact gets blinked out of existence. And even then it's debatable. Qs operate on an entirely different level. They have complete control of space, time, matter, energy, along with reality warping, TK and TP abilities. They were also shown to have a 2,000 + IQ in mortal form. Qs are stated to be: omnipotent, all seeing and all knowing. They've blinked races and worlds into existence just to make a point to Picard and his crew.
The only reason the Voyager crew was able to enter the Q Continuum was a female Q helped them. Thanos isn't just going to trick a Q into a giving him a Q weapon. And Thanos certainly won't trick a Q into any half baked schemes of his.