I can only answer 2 and 3.

In my experience (but with AONEs) you pay the price at the time you issue the ticket. If you want to lock the price in you should buy now. You need to pay when the ticket is issued. There is a risk that the issuing airline goes under before you finish the ticket and you would loose some or all the money - as is the case with Air Berlin).
Also some airlines cancel reservations if tickets are not issued a certain time after the reservation is made. This was for A class, but it was only a few weeks.

I know that several people where not using sector(s) at the end of the ticket without problems.