Like others have said, you would probably have an easier (and more fun) time finding out in character. As for what your character would observe, I would highly suggest clearing stuff with a DM first and trying to FOIC firsthand. You may even be able to arrange a time with one to observe behavior in game. I also recently finished my portrayal of my wererat AMPC and you won't be hard pressed to find people who've done some research on him or have observations to give.
1) My wererat came out at night because the moon was a trigger for him, though not all wererats have lunar-based triggers. It's also advantageous to sneak around during this time. They're opportunistic, clever, and certainly not suicidal as many guards patrol the streets in the day with silver weapons. If the average commoner crossed a wererat in the sewers or at night, they probably would end up being eaten, robbed, or both. They wander about for many reasons, since they're vastly varied -- food (people), theft, curiosity, and territory. There is also an element of mischief that I had integrated into my AMPC as well. They're as diverse as people are.
2) Since wererats often compete for the same sources of food as vampires or other undead, they tend to stay away from them as they're weaker. If the Count was walking about (highly unlikely, by the way), a wererat would likely not cross his path and would likely hunt elsewhere. That said, each wererat would respond differently. If he was dressed in valuables, a more daring wererat might try and pickpocket him (and probably not fare as well).
3) Wererats actually aren't all that strong. As for home invasion -- it wouldn't surprise me if it is technically more prevalent when you pull back the curtain of the "NPC world" vs. the "PC world." Though, numbers, light, and locked doors would easily deter a wererat, who would better spend that time looking for an easier score.
4) Wererats avoid crowds and well-lit places. If they can't sneak around and skulk the shadows, they probably won't go there. They also aren't accustomed to going someplace crowded or enclosed, where they can be encircled or trapped. While it doesn't answer what places wererat's avoid, they are territorial and would probably prefer to stalk out an area marked out as their own.
The link posted by Myrza will probably cover most of this, though there is much more nuance not included that you'd best find out in character.