I've never been to Keng Tung - on my to-do list along with Mong La over the chinese border there but now I hear you need to hire an official "guide" to go there. I'm waiting for Myanmar to liberalize more - it will happen by 2015. Just last week, Myanmar changed its forex BS policy so miracles do happen. In the future then, you can cross over to Tachilek, go to the airport (yes, there's an airport there) and hop on a flight to Bagan, Heehaw, Mandalay etc
I don't speak Tai Yai but I can guess when someone's speaking it. I think over in Tachilek, most of the people there are not Tai Yai but Nuea. You must've not been to Tachilek in a while - I believe the casinos opened in 2001. Anyway no loss, Tachilek while interesting, is "1-day-only interesting". And dangerous - until 2002 I believe, there was an active war going on there between Myanmar, Thai army and the Wa. Several bombs and mortar shells went off in Mae Sai. A couple of soldiers killed as well. The Wa is still there.