Absolutely! Their list of tastings mention which ones require reservations, and those that don't. https://www.chateaustjean.com/visit/daily-tastings
Yes! They welcome you with open arms.
I've been there 3 times and the 3 times without reservations.
Yes, you can come for tasting without a reservation.
Yes you can. I have went many times without reservations. I am a wine club member though, but you should be fine without reservations.
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