KJV Wimps?

Probably not. There are two different readings of Job 13:15, one is called the qere and one is called the ketib or ketiv. The ketib is what was written in the text, and the qere is a marginal note made by a scribe who copied the ketib. Marginal notes were made for various reasons, including when it was suspected that a prior scribe made an error. Suspicion of error could arise on many grounds, including variance with the traditional reading, etc.

In Job 13:15, the ketib has lo' meaning "not," and the qere has lo meaning "in him." The scribe who made the qere thought that "in him" was the better reading, but copied from a text that said "not." So, in the text he made, he put "not" in the body and "in him" in the margin. Whether "not" or "in him" is really the better reading is potentially up for grabs. Most modern translations go with the qere rather than the ketib on this verse, as did the KJV translators.

But I find it hard to believe that the KJV translators nervously misread the text. That kind of accusation is unfounded. They appear simply to have gone with the qere, believing that to be more accurate. In fact, they did this frequently. Besides this, they faithfully translated many other passages in which Job questioned God even more directly, and the immediate context of Job 13:15 fits well with the qere reading. All in all, it looks like a cheap and unwarranted shot at the KJV.

In any event, "I'll not quaver" may not be a great translation since the verb in question more typically means "hope" or "wait." Even the liberal-friendly NRSV (which I happen to like quite a bit in most cases) reads: "See, he will kill me; I have no hope." "I'll not wait!" might also be a decent translation (as in "I'm impatient!"). The NRSV also contains a note with a translation of the qere: "Though he kill me, yet I will trust in him." Most other major English translations follow the qere.