] At this point in the text we have only Mr Wickham's word for it( for what that is worth), don't we?...

] together with and these toadying hints put forward by Collins to Lady C and Anne....

] I am not sure Darcy knew of this arrangment.

If only Wickham were spreading the story, it wouldn't have much credibility -- though it's a little hard to see what he could hope to gain by that particular lie. (His other lies generally have some discernable purpose.) However, it seems to be stretching coincidence a little too far that Mr. Collins came up with the same story, completely independently, if there weren't some basis in fact. (The "fact" meaning that Anne and Darcy were expected, at least by some people, to marry, not that there was actually an engagement.)

Also, their sources were very likely different. Mr. Collins knew, it was most likely because Lady C told him. On the other hand, Wickham has no known connection with Lady C. If Wickham knew, it was most likely because the possibility was discussed generally enough at Pemberley that the son of Mr. Darcy's steward knew. If so, this likely came from Lady C's sister, Darcy's mother.

On one hand, if it was that generally discussed at Pemberley, it seems unlikely that Darcy didn't know anything about it. On the other hand, he would not have proposed to Lizzy if he had considered himself engaged, or in any way "bound" by this arrangement.