Do you think the Tyrell plot to marry Sansa to Willas was serious, and could they have really pulled it off if Sansa didn’t spill the beans to Ser Dontos? What would have been the Lannister response?

I think it was pretty serious. The opportunity to take over an entire Kingdom of Westeros is way too good to pass up, and once Sansa is out of King’s Landing, the Lannisters can’t really do much publicly besides put a brave face on the situation.