unheard

There's no much info on passive cooling out there. Australia seems to be the only first world country that's is actively promoting passive cooling designs. The rest of the first world countries mostly reside in temperate zones where cooling is not a priority.

Could you elaborate on the ash problem? I don't quite see why it would be such a big obstacle for including a ventilated attic, even in close proximity to the corn burning areas.

Yeah, it's a balancing act based on personal preference. Plus other factors like house orientation, number, size and position of windows in the particular room(s) where you spend most of your time, wall shading etc, etc. You might consider installing double glazed windows in just one bedroom and(or) living room as a compromise. Or to take advantage of the much cooler night temperatures by creating ventilation paths inside of your house (depends on the geographical location) plus ventilated underfloor. Your proposed design has huge glazed areas that would be more expensive to insulate than a house with much smaller windows. You might want to re-think the design if thermal qualities is a priority. Otherwise it's gonna cost you.

Render finish, not structural cracking. Never a problem? ????

You're arguing apples vs oranges. Yes, AAC block can be layed down faster. But... Have you tried to price a brick laying job with AAC block vs Red brick? Red brick is still the default wall material in Thailand! Every brick layer knows how to do it right. AAC blocks is a more expensive option since it requires a specially trained worker to be able to get it right, including rendering. Trained as in the ability to build and finish the wall that doesn't have the constant cracking problem few years down the road.

Yes, you'd need to consider all elements of your house design and decide for yourself how much you'd want to spend on improving your house thermals. Like in your situation with solar... It probably won't be cost effective to go overboard with better insulating materials. Walls, windows, ceiling, roof, doors - all contribute to thermal energy transmission. Going "standard" would save you tons of money. But if your goal is to insulate every major contributing part then your overall house price will dramatically increase. As in regards to the highly recommended AAC blocks. Yes, they're great, especially in thicker sizes. But, you'd also need to consider the extra costs. If done right a wall constructed out of AAC bricks would be considerably more expensive labor-wise as compared to the standard red brick wall. You'd need to go with an experienced AAC block layer or risk a possibility of the constant render cracking later on.

You wouldn't need a thick insulation layer if you'd opt for a PU insulated metal roof. Yes, the metal roof looks are not for everyone but then it all depends on one's priorities. It seems for most looks almost always win over thermal insulation.