For whatever reason, I can't log in to your blog... and thus can't post. Here's what I wanted to say, though:
Check out "Descent of Angels", "Fallen Angels", "The First Heretic", and "Aurelian" once more. The Dark Angels novels pretty much nail down Lion El'Jonson's time of arrival on Caliban and time of discovery by the Emperor relative to the beginning of the Heresy. Similarly, either "The First Heretic" or "Aurelian" gives a solid indication of when Lorgar was found. I'll give you a definitive answer after some errands, but I'm fairly positive Lorgar was discovered BEFORE the Lion.