I'm being a bit of a Sourpuss again sorry but Prospero is literally on the other side of the Galaxy (Ultima Segmentum(east)) to Colchis (Segmentum Pacificus(West). That's why it was such a big deal that Magnus managed to beat Lorgar in the quota because he's literally on the opposite side of the Galaxy when he gets found with a legion that has less than 10,000 marines in it and he's getting more planets into compliance than Lorgar. This all occurs 100 years after the start of the Crusade.It depends, if he gets a new fix on Lorgar's location he may change his destination to wherever he is and never go to Colchis.
It depends on how noisy Lorgar's activity is when he's off-planet. For example, if we somehow make a beeline for Prospero and then Lorgar and Thalassa help Magnus make his own Sons, then detect and defeat the Fleshchange, that's probably a big enough deal that he'd head there instead.