The Thai pass is nothing more than a pre entry clearance. It serves no purpose after you've entered and has no bearing on the length of time you may stay in the country
Tylenol PM is basically paracetamol mixed with the antihistamine diphenhydramine which makes you drowsy. Both can be purchased over the counter very easily
A confirmed flight itinerary should suffice. You'll find websites offering the service for about $15. I used a company called travelvisaguru and had no issues