Excellent summary here on this important topic, and would just add that when those against the CT claim that it is subtracting from the text, but that is based upon their assumption that the TR is the exact copy of the originals to us.Tom, the short answer to your question is that if your definition of "pure" has to be taken with regard to the WS, then we do not have God's Word at all, unless you are willing to claim that the autographs are 100% equal to a manuscript that we currently have. I am not willing to say that, and Stephanus was not willing to say that, since he based his edition on MORE THAN ONE manuscript. Logan brings up the other important answer to your question. The Westminster divines were almost certainly responding to RCC claims that Scripture had been badly corrupted in the manuscripts (hence the essential need for the magisterium). In that historical context, the Westminster divines were responding to the claim that the text was so corrupted that people couldn't read the Scriptures on their own.
Your analogy of a drop of ink doesn't wash, if you'll pardon the pun, because it makes the mistake of the word-concept fallacy. What I mean by that is that word-order differences, different spellings of the same name, removal or addition of particles that often don't affect the basic meaning of the text would not constitute, any of them, corruption of the meaning of the text. The meaning of the text remains intact, and therefore pure. So, when I say that the TR is 98% pure, and relegate by far and away the majority of the remaining 2% of the differences to non-meaning-changing differences, I can still say that God's Word remains pure. By the careful comparison of manuscripts, we can arrive at the original reading in almost all cases.
The problem here is that text-critical scholars often use the term "corruption" to describe something that is only a difference between two manuscripts. Then, when people see the term "corruption," they get really uptight about it, and start imputing intent to corrupt to the poor copiest who was only trying to do his best. It is one reason why I prefer the simpler and less fraught term "difference" to the term "corruption."
One other thing needs to be kept in mind, folks, and that is that saying "omit" and "add" prejudges the particular reading, as if a standard is already assumed, and manuscripts that change anything from the already assumed standard are corrupting the text. If there is a difference, it is more accurate and less prejudicial to say "plus" for additional material in one manuscript and "minus" for less material in the other. That way it is not prejudged whether one manuscript added something or the other took it away. Lack of caution on this point is particular bad in TR advocates, I have noticed. They simply assume that the Alexandrian texts "omitted" something, without considering the possibility that the Byzantine manuscripts "added" something. Each reading must be considered on its own, since each reading has a completely different set of manuscript witnesses to it.
We do not even have today settled as to what would be the true and real TR text, as Erasmus used 6 of them, and the one most seen today as being that is the 1894 Scriveners, but there is some doubt as to its full validity .
We do not need any Greek text to be 100 % exact copy to the Originals themselves to be seen as the word of the Lord to us, and the TR/MT/CT all can be seen as being that to us in the Koine Greek for today.