I think you missread my post a bit. I actually think (that's of course only an assumtion) that you have to be for Emma on nobodys other route. If I remember right with Kogas mod showing the infos you get opt out for the Emma dream when you are on Charlottes route. And it will imo also be checked for Emma later on if you are on other routes, too. But we will have to see how DB is handling that for real in the next updates.
I didn't misread your post. I'm only trying to interpret what the dev could have meant by "it won't be easy to reach the Lucy route". It's pretty certain that Emma's route would become exclusive at some point, as it will be for Lucy, Suzy, and Charlotte (and maybe, a few others too). You have to be on nobody else's route to end this game with any of them as your chosen LI. But if you could still reach Emma's route by following Charlotte's route, that makes it a no-brainer for anyone who wants to attain the Emma route. You simply have to go on choosing Emma over Charlotte, Suzy, etc when the time comes. However, "it won't be easy" if, say, when following the Charlotte route, you don't even get the option to get the Emma route at all. It will be difficult when you must choose Lucy over Charlotte at the first fork and then, go on rejecting everyone else when the choice to romance Emma opens up and that becomes the only way to do it. Of course, these are all assumptions (I don't own an apartment inside the dev's head) but I won't mind that kind of difficulty for a change rather than just going along with the Charlotte route to find the Emma route open up at some point when all she's been feeling towards the MC is a growing fatherly sort of love so far. And even though this is an AVN and we've seen far more depraved plots, I think getting Emma through Lucy's route would be a lot more believable to me. I'll take that as a welcome choice.