To the points you listed:
1/3: I've always thought that the girls falling for the MC so quickly was due to his currently unknown origins. Like it's a side-effect of whatever is going on with that. The feelings aren't fake or forced, they just happen a lot quicker because of it.
2: With how many girls there are and the amount available of time each day, you can only spend so much time with each of them. All of them understand that they have to share the MC though, so it shouldn't be too big a deal to not spend time with every girl every day. Newer girls are of course going to have less time since they only recently get introduced, but they'll get more as the story moves forward.
4: With him being 500 years old doesn't matter. That's applying short-lived race logic onto a long-lived race. Aliza is the same age and in the same boat as the MC. They just work differently.
5: Everything has made sense so far and the story flows well enough.
All-in-all, no real complaints about anything thus far. Keep up the great work!
(Don't worry too much about remaking things.)