Are the tickets on sale every 10th of the month from the website?
Unless you are able to pick up tickets at a Lawson's on the 10th of the month prior to your visit, you'll have to order online through a company like Viator or Japan Awaits. For this, you need to order by the 5th day of the month prior to your intended visit I believe.
No, only in person at a convenience store in Japan. That's why I went through Viator.
Viator are facilitators. There are a limited number sold by the museum, and if you live a busy life, Viator will make the process easier.
Get an answer from Pre-Order request for Studio Ghibli Museum – including Ticket delivery staff and past guests.