@Brittney Lundeen this is a tough spot to be in and COVID-19 and eviction rule changes makes it even more challenging. Especially in MN we tent to be very accommodating, but remember that this is a business and your investment is at stake.
I'm not a lawyer, so these are just suggestions, but I am a fan of being as explicit as possible in my leases. If not in there already, the next lease you write up should specify when someone goes from guest to tenant (4 nights/wk, is on site more than 6hrs/day...or whatever you think) so you remove as much subjectivity as possible. Even if he was a perfect guest, if he's there all of the time he is contributing to the wear and tear on the property, water use and utilities (even if tenant pays utilities your mechanicals are working more) so you should be compensated for that. Not quite the same, but if they were running a business from your property and 20 people showed up to work every day but left at 5 and didn't sleep over you'd want to change the structure of the lease.
Similarly, build in specific consequences for smoking in the unit. Fee of $X for cleaning fee or HVAC filtration (if applicable) on first offense. Forfeit of security deposit on 2nd offense. Termination of lease on 3rd offense. Again whatever you feel comfortable with in terms of escalation here.
No personal experience with this device and not sponsored, but I've looked into FreshAir Sensor previously for my Airbnb, and it is a simple plug and play device that detects cigarette smoke. You'd be able to collect empirical data that someone was smoking in unit or basement.
Last tip, get it all in writing. Even if you have an in-person conversation (from 6ft away), write a follow up email and say "Here's what we discussed today. No smoking. The next violation will mean I keep $X from your deposit for deep cleaning/paint. Guests are tenants if they stay over more than Y nights/week and need to go through the application process. etc. etc."
Good luck!