There are a couple of current threads related to this topic - there currently does not appear to be any confirmation of fares going up but there is also an apparent "10% sale discount" starting on Feb 1 - so as someone has already pointed out, maybe the fares will go up 10% on Feb 1.
If you buy it online using the oneworld tool, then presumably there should be no issues in getting country-of-origin pricing.
$125 each time you change destinations. No fee for date/time changes. Best idea is to pick the places you currently think you want on the ticket (and specific dates and times, as you can't do an 'open' ticket online) and then change them later if you have to. It's $125 for each set of changes, not for each changed destination, so if you are going to make changes to the destinations, try to do them all at the same time.
I think the 12 month validity of the ticket runs from the date of the first flight and not from when the ticket is issued but perhaps others can confirm that (as I've never done a ticket that I wanted to go for more than 12 months out from when it was originally issued).
Thanks tt7, thank you very much for your clear explanation, much appreciated.