I really don't want to get into what could be an endless debate because ...
(A) ... it's hypothetical. We don't know the answers to many of the questions we're posing. Also, ...
(B) ... my time is limited, what with a 3-day old baby in my household.
Let me at least offer you this, though - just as food for thought, not as a rebuttal. I have no idea how much knowledge of the Webway the primarch in question would have prior to invading Commorragh. For this debate to even work, that question would need to be answered first. "None", "partial", and "total" all point to three completely different answers to this scenario. Hell, "partial" could encompass anything from "just enough to get in trouble" to "enough to ruin the Dark Eldar for the foreseeable future".
And, probably just as important when it comes to dealing with the unforeseen circumstances that the Webway can impose on travelers: the Adeptus Astartes defend the length and breadth of the known galaxy by means of a travel mode that involves going through Hell. The "sea" their starships "sail" on is sentient, and everything in it wants to kill them. The Legiones Astartes conquered
the Imperium the Adeptus Astartes defend, under very similar conditions. I pose to you that Commorragh, despite being a place of enormous complexity and danger, does not exceed the size of the stellar territories a Legion would have conquered during the Great Crusade and mathematically cannot be more dangerous than the Warp and all its creatures, which it was constructed to avoid specifically.