First off, this is the first I've heard of Pothead Books and it looks absolutely hilarious
On topic though, it seems like Dana would be the easiest to ask some questions. He seems particularly open, not to mention that the other psilocybin chain sounds pretty serious - partners and legal teams and all that... if you're willing to follow up with them, be my guest, but it might be more work than it's worth.
Either way, it seems like there's a lot to say about Dana, so that might be a good place to focus our efforts. From my brief investigation of him, it seems like he's said a LOT about weed, so investigating his views and plans for mushrooms might be more original and more fitting for the topic of the magazine. Perhaps you could ask about what dangers running his business poses, why he's so passionate about this, if he plans on expanding to more drugs/what his plans for the future are, and overall his insight on the current state of drug culture in Canada. Maybe even how he thinks others could take the initiative to expand this movement! I'm sure he'd have a lot to say.
I think it's better for the scope not to be
too large. I think an article focusing on the emergence of psychedelic shops in Canada is timely and interesting enough for a full length article! And who knows, if Dana has enough to say, an article for a future issue might be in order.
Are you thinking to do interview(s) through email, over the phone, in person? Whatever your method is can change the way you'll phrase your questions. Again, thank you so much for taking initiative! You might be jumping the gun, but jumping the gun as a group is how stuff gets done. Keep me updated on what you're thinking, I'm happy to contribute to this in any way I can and excited to see where this idea goes.
(Don't let me deter you against the other chain though, I think a more professional business could provide some very interesting information and answers; just do whatever you're willing and comfortable with doing.)