Given your situation, yes. Lukic is more of a Utility and given what I've previously said about Utilities, Boy is probably a better option, and Lukic will be better off coming off the bench at this stage IMO. Not much in it, and ideally neither would be starting First Grade.
I'd have thought it was a bit of both. Downes post count is below 200, so is probably out of his depth. But as you say Marcuss is also very very good.
This. Marcuss is the Seasonal Rugby Test #8, you're a reserve Flanker and in all liklihood would be a reserve at every club. Effectively, you're replacing an International with a Club player. Though, I actually think you'll go okay, it does make a significant difference to the odds. If David Pocock was replaced for his Super Rugby team by an unheard of reserves player, it would impact the odds I suspect.