]If Darcy and Elizabeth had already been engaged when Elizabeth felt unwell, then I think he would have had the right to go and inquire about her as well, but as it was, he didn't have the right to visit a marriageable young woman he was not related to, without raising a lot of eyebrows.

I had an answer for you about what Darcy would or would not do in answer to your suggestion that Darcy would not let his feelings overrule his right to discretely interfere in situations when I realized we are on the group read board and that information is not yet available to us. But here is more speculation for you. The Collin's servant(s) could well have come recommended by Lady C. and therefore better known to Darcy than the Collins' themselves. He may either have been well enough aquainted with them to feel he could depend on their discretion or conversly to know that they were not as dependable or sensible as Lady C. adjudged (she did pick Collins, after all). I don't think that Mrs. Collins, as sensible as she is, would have much choice in picking out her own servants over the recommendation of Lady C.