I HAVE THE SAME THING. PROSOPAGNOSIA. I thought this was perfectly normal, and was very puzzled when I saw police TV shows where they ask potential witnesses about the criminal's facial likeness. I was always like, "but how are you supposed to remember that unless you have superpowers?"
With my current comic, I have it relatively easy because the main character is of a minority race (white male, haha, but he's in a place where everyone else looks Asian). It's easy to make him look distinct with his big nose, sturdy jaws and butt chin. I even gave him huge bushy eyebrows to top it off!
Still took me a while to figure out how to draw him right, of course, but I think I've done well considering everything.
The weird thing is, once I took a portrait painting class (not much instruction though, just a time and place where you get to sit and paint a live model) and my paintings almost always nailed the model's likeness. I couldn't tell, but a lot of people commented on it.
THIS TOO! Well, I can't always do it this way; sometimes I'm not ahead enough to be sketching 10 updates ahead because I have to finish the next update, haha. But when I have the time, I definitely prefer it this way. At the very least, I like to get the thumbnails for the whole chapter done before doing any other work.
One other thing regarding consistency: often you're the only one to notice the inconsistencies. Even if they look glaring, other people may not really notice it. I believe in focusing on the basics before worrying about the style. Unless you're doing something drastic like switching from hard-lined B&W to lineless full color, you're probably fine. 