Can you continue playing after you finish the main storyline

I'll do you one better.

Rent levels do not effect your morality over time.

Instead you get dinged initially (when you set it), but when you change it back you can get that morality back. When I tested it, I initially set my properties to high (as I needed money at the beginning) but once I owned all of Albion I pushed it back to low; and my morality came back.

Now I can't be sure it was in equal amounts but it was more than enough to get me over 100% long before the end.