With the third case the possible use again was stated as conditioned upon it becoming legal at the Federal level. I see nothing wrong with that. This person says if it were no longer illegal he might do it. If it is ever legal at the Federal level then I do not see a problem with this person then using it again.
As far as people living in states where it is legal using it and that being a problem getting a clearance down the road, the government needs to change the policy. I know that there are officials very aware that the current policy is a problem. If someone doesn’t already have a clearance, chances are they do not realize that there is any legal consequence to using a substance that is legal to use at the state and local level. Using Marijuana legally is not an accurate prediction that this person would break the law knowingly in the future.
However, I do not believe anyone with a clearance currently can truly not realize it is still illegal for them to use Marijuana under any situation.