I've gotta dash, but essentially, the fundamental linguistic premise behind 1984 is this idea that, if people do not have a word to describe a thing, then they cannot meaningfully think about it.
This is, to put it simply, just not true. The greater concept is called linguistic relativism, or the theory that specific languages play a significant role in our general cognition, but outside of some very minor effects, evidence simply doesn't support it. All human languages are essentially of equivalent complexity, and even in situations where a pidgin is created through language contact, it rapidly re-complicates into a fully developed language.
For a concrete example, the idea is that, by replacing 'bad' with 'ungood', people's domain of thought will be meaningfully reduced. The problem is minds don't actually give much of a shit about etymology. In practice, what would rapidly happen is that 'ungood' would come to simply be the word for 'bad' just as deeply as the word 'bad' is to us. To give you an idea of what I'm talking about, consider the word 'discover'. When I say it, you might think of a new scientific discovery, an explorer finding new land, or something to that effect. What you probably do not think of is that is quite literally 'dis-cover', that is, to undo the act of covering something up. Etymology very rapidly gets disconnected from peoples' internal sense of a word, and to that end, manipulating it doesn't really do all that much.
To go back to Newspeak, it's trivial to re-develop a word for 'rebellion' with something like 'goodthink freeness', which will quickly be internalized into meaning the same concept. The range of possible thought doesn't actually get meaningfully reduced.