Can you mix ECC and non-ECC, yes, but its highly discouraged. Can you have non-ECC entirely? Yes, it's like putting 87 octane into an engine that takes 93 octane. It'll run, but not to full performance (in this case the error correcting code feature won't work)
Pretty much. Nothing is ever free. You want error correction you pay a little in ultimate performance. A couple % won't be possible to tell the difference in day to day use so its probably not worth worrying about. If you are doing weeks long calculations that can all be wasted if a bit is flipped you can bet it's worth way more than the little performance hit.
I expect 1866 ram prices will drop after the initial "goldrush".
It is always like this when the next higher speed tier solidifies.
I recall similar issues when DDR400 replaced DDR333 on mainstream PCs. Couldnt even find DDR400 sticks in any size but 512mb for a bit.
Likewise there were some shortages when DR1066 was coming up as well.