The slippery slope in Darwin-plagiarism theories

The Slippery Slope argument is a logic fallacy (a non sequitur), in which the proponent argues that, once the first step is undertaken, a second or third step will inevitably follow, much like the way one step on a slippery incline will cause a person to fall and slide all the way to the bottom. However, this sort of thinking does not allow for any possibility of stopping the process. It simply assumes that, once the nose is in, the rest must follow--that the sheik can't stop the progression once it has begun--and thus the argument is a logical fallacy.

One of the most disingenuous strategies employed by Darwin-plagiarism theorists is the following.

1. Evidence on the reception of Matthew (1831. On Naval Timber and Arboriculture) by various contemporaries is taken for a claim about the likelihood that Darwin and Wallace had heard of it before 1858. For example: "It is more likely than not."
2. From there you slide to claims about the likelihood that they had read it before 1858.
3. And from there you slide to claims about the likelihood that they had received the idea of natural selection from it before 1858.
4. You then replace the likelihood claim with one of absolute certainty ("100% proven"). But you change the content of the claim, underhand, from Darwin and Wallace having had knowledge about Matthew (1831) prior to 1858 to various contemporaries having read and reviewed Matthew (1831) before 1858.

Here it reads like this:

"... evidence - that it is now more likely than not that Darwin and Wallace plagiarized Matthew's prior published hypothesis and that each lied when they claimed no prior knowledge of it."

"Wallace and Darwin claimed not to have read Matthew's (1831) book and have been newly proven 100% wrong (by me) in their claim that no naturalists known to them read it!"

Did you notice how the content of the claim changed underhand?

From: "Darwin and Wallace plagiarized Matthew's"
to: "Darwin and Wallace claimed not to have read Matthew's (1831) book"
and from there to: "their claim that no naturalist known to them read it!"

Or did you instead gather that the plagiarism is now claimed to be 100% proven?