I'll agree with the other that all that flying around to spend 2 days each in very similar type beach-y places doesn't make a lot of sense to me. But I am biased against beach-y places for the most part.
Personally I would head up to Chiang Mai and the north instead. I have never been in Thailand for Songkran. I have read plenty of opinions about it. It sounds like it has basically become a crazy free-for-all of the biggest possible waterguns to shoot everyone with. Doesn't sound like the sacred and beautiful holiday it used to be. But from what I read Chiang Mai is a better place to be for it. I'm sure dsquared will have plenty to say. :-)
I spent a couple of days in Khao Yai last November. It is an enormous park and big area. I'm not sure how you are planning to get around. It's not like there is a lot of public transportation out there. I'm guessing the weather will be HOT. It was pretty hot even in November during the day, although it was quite chilly overnight and in the early AM. Also a lot less English spoken out there than in the typical foreign tourist areas. Is your 4-5 days Khorat in addition to the 2 in Khao Yai or including? I did a little trip to Isaan a few years ago and we stayed in Khorat as the base for day trips to Phimai, Phnom Rung, etc. I think 5 days is probably too much and there's really not much of a town to stroll around. But it wouldn't kill you I suppose :-).
I'm not sure your trip is the kind of trip I would take. But that's neither here nor there, I suppose. I would probably go to Cambodia instead of taking a multi-day bus ride all the way down the peninsula.
Don't worry about Thai people's attitudes. Most Thai people take hospitality very seriously. Be sure to read up on the scams that are out there though...especially in Phuket which sounds like it has more than it's fair share. But I have found that people all over SE Asia are incredibly friendly and nice in general.
Enjoy your adventure.