Yes, you are correct. DONE4 must be OW metal.

You can change date or time without charge. Adding a flight or otherwise rerouting is US$125 + extra taxes + possible service fee. Obviously the reissue must stay within the rules.

Yes - there needs to be D availability (and as far as I know all D inventory is equal). The reservations can be largely separate from the ticketing - you can have up to 20 separate PNRs/reservations corresponding to a ticket. Depending on airline policy you simply need to make sure the ticket number is associated with the reservation to be able to hold it beyond a certain point (with some airlines that will be "there must be a ticket number there within x days of making the reservation", with others "there must be a ticket number there x days before the flight").