I am not real familiar with the Skyteam Europe pass but I am with the Star Alliance one. Generally you have to set your route in advance, the price is calculated based on the number of flights (coupons) you use, though it sounds like with Skyteam it is a combination of number and distance of the flights. Changes are generally allowed but usually incur a nominal fee (with Star Alliance, date changes are free, routing changes are $50).

The reason you need at least 3 and less than 16 coupons is because those are the rules they set. Same with RTW tickets - they have rules too. My guess is that the 3 coupon minimum is so that someone doesn't use it just to replace a simple round trip ticket. Skyteam is more generous than Star with a max of 16 flights and also more generous than Star with allowing 2x visit to each place (Star allows just one). This is so you don't "base" yourself somewhere.

Most likely if you don't use the coupons you get little or no refund. Most passes like this that I have seen say something like if you don't use the coupons, your fare is based on the "one way" fares for the coupons you used and your refund is the difference between what you paid and the "one way" fare for each flight you took. It doesn't take a brain surgeon to figure out you probably get no refund after calculating the one way fare for just a segment or two.

Also, most likely you can use OK as well, maybe even SU, RO, AZ.