Welcome - you've seen the board in full flow already, I think ;-) Your boys sound fun and yes, many of us have btdt. My 2p-worth:
Preschool: question 0 is, do you actually want or need to send your DS3 to preschool at all? An alternative might be to skip it and just make full use of playdates, playgrounds, clubs and whatever else is available to give him the chance to play with other children.
Question 1 is what's the language immersion one like? Some people have had good experiences with such, the idea being that at least your child is spared pre-reading instruction in English and does get to learn something else, i.e. the other language. Opinions differ about how useful bilingualism is (in terms of brain development, regardless of whether one ever uses the other language) but at least it'd be likely to be a positive.
FWIW my DS8 had a brilliant time in a completely play-based nursery (preschool), and I formed the opinion that the less they try to teach the children the better :-)
IQ: I think most people will suggest leaving it until your DS is 6, unless you have a pressing reason to test earlier, and then the most widely recognised test is the WISCIV (or actually, it'll be the WISCV by then I guess). Consider then whether testing will actually be giving you anything worth the very considerable cost; it doesn't always. (My apparently-very-unusual DS8 has not been tested, and we - parents and school - discussed it again recently and concluded that it wouldn't be doing anything except satisfying our curiosity, so we won't. In the US, a high score would open DYS to your DS, of course, which might be sufficient reason.)
Schools and areas: I'm not in the US and my US geography is hazy, but the name that leaps out from your post is Reno, because of the
Davidson Academy there. That's for wildly extreme children, and really honestly you can't yet be sure whether or not your DS will be one of those, but it's something to bear in mind.
You talk about public schools: so are private schools likely to be out of the question? That obviously gives you less choice than if they could be considered. How do you feel about homeschooling and how it would or wouldn't fit into your lives? If you'd be fairly happy to have that as a backup, then schools become a bit less crucial.
