Gaseous Form

Looking for some discussion or references re Gaseous form. Had a scenario last night in a game where an assassin drank a potion of gaseous form to flee the scene. Group had some pretty good responses and I didn't have answers. My questions are a) can someone in gaseous form be polymorphed to a beast? - Polymorph can turn a creature into a beast. Gaseous form turns a creature into a misty cloud - Is it still a creature? b) next question, and I am going to predict the answer is yes, can the mist be attacked? I assume yes, because it says if creature drops to 0, the spell ends.  Be an interesting outcome, because currently the assassin has 2 hit points and is about 12 in the ground, going through cracks etc to escape. The group are using shatter to attack it underground as the spell says a point of your choosing, with no parameters about being able to see etc. So i guess if he dies, he will re solidify 12 feet underground. 

The spell specifically excludes shape changers from the effect of Polymorph. Such as a Vampire that can change into Gaseous form. Are you suggesting that if a creature is in gaseous form, and it was changed into a bear, it would be a bear in gaseous form?

I should specify, that we are staying with 2014 rules. Took me 37 years to move from Advanced and 2e to 5e. Don't plan to move forward.  Agreed about the shapechanging issue. They creature would have to have it as an innate ability to be considered so.

If you polymorph a creature under the effect of bless or aid you'd reasonably expect them to still be under the effects of the spell. The same applies to gaseous form.

Since the potion of gaseous form does not require concentration, the characters end up with a gaseous beast unable to turn solid. A probably very confused beast. :-)

If the spell landed, anyway. Polymorph requires the caster to see the target. If the assassin had already escaped into cracks in the ground then they couldn't have been seen.

First round he was seen, which is where the idea came from. It wasn't until the second round that he fled into the cracks of the earth. Then they used to shatter, with the focus point below ground to cause further damage. Pretty good outside the box thinking, I thought.