Presumably derived from the activity of good (as in effective) politicians.
Rather than just using DoubleSpeak? (saying something which they can later say means something else) they go one step farther and actually believe what they are saying, even though they know that in future, their interpretation will change.
NoNoNoNoNo you fool. That's not doublethink. Doublethink is the act of simultaneously believing two completely contradictory and mutually exclusive things.
And seeing no contradiction.
There's no 'in the future' about it (indeed, any doubleplusgood newspeaker would be appalled at any mention of any time or tense other than the present).
Not at all. "He who controls the past controls the future, he who controls the present controls the past." History is an important part of the control of the Party in 1984. And I'd point out that a doubleplusgood newspeaker could easily be crimethinkful and have doubleplusungood views about the past, provided they knew oldspeak as well.