I'm sorry about the frustration regarding the change in our pricing model. I see that Blake already answered your general questions in a ticket, but to give you answers here as well:
1) You could do that provided they deactivate the DVS license on their computer(s) first.
2/3) Yes, any hardware that unlocks DVS would work, even in combination. The hardware would also provide a one-time purchase price.
4) $1000 on subscriptions? Even if you had the Creative plan at full price ($15/mo), that would be over 5 and a half years before you hit that amount!
Listed hardware will provide DVS functionality so long as you keep using it with the current rekordbox version. I don't know what v7 will hold, but by then, the DDJ-1000 should be pretty old!