Here is an updated plan with heights of the knee wall and ceiling.

Thanks for your suggestion but I do not want a second dedicated switch for a fan. And no, I'm not making this waaaaaaaay too complicated. You guys are making this complicated by trying to get me to not use a motion activated timer. That's the non-complicated way. I don't have any special issues and my friends don't really stink. If you don't care about this stuff for YOUR bathrooms then there is no reason for you to judge ME for wanting to do it in the best way. Even if you don't think I need to do all this, I didn't come here asking if I SHOULD do it, I am asking for HOW to best accomplish my objective. The problem is that product I want doesn't seem to exist. 50 CFM with sufficient volume and a way to keep the fan running for a few minutes after the visitor departs. Motion activation seems to be the way to accomplish this. Seems like I can get fans with two of the three features in various combinations but not all three in one. I want my bathroom visitors to turn on the light and I want the fan to work automatically, not on a separate second switch. Based on my research the solution is to buy a 50 CFM fan that is sufficiently loud then take care of the fan activation with a motion sensor. Not complicated. So again... 1) Can someone give me some advice as to how many sones I should get? I'm 80% sure it should be 3 or 4, but 4 is twice as loud as 3 so some guidance on this would be helpful. This is my most pressing question right now. 2) I've identified a Broan and an Air King that meet the criteria for the fan. I'd like to know if anyone has experience with these brands and are they reliable? 3) As an alternative, I can get an 80 CFM fan that has everything in one unit but from what I've read I'm pretty sure 80 CFM is way too strong for that size powder room. Anyone think 80 CFM would be OK in a room that small? I don't think that's so complicated. There is a fourth question I'd love to know about which IS more complicated but you guys brought that into the fold. 4) Is there a way to get the fan to come on with the light switch but stay on for several minutes after the light switch is turned off? I do not want second dedicated fan switch/dial/etc. On a side note, an occupancy sensor should not be finicky here. It's a tiny room with one way in/out. I think it would be hard to put a motion sensor in this space that would fail to accurately detect an occupant. Thanks guys.

Is it possible to relocate the bathroom? Could you move it to where the laundry room is, and then have that back hall be a mix of laundry room and mud room? You could also change the door location so that you do not have to go through the kitchen to go into the mud room. Rough sketch -- not drawn to scale:

Since the vanity is between two walls, I'd purchase a cabinet with appropriate filler to fill the entire space, then have a counter templated. I wouldn't want the vanity floating in the hole between walls. Build it in, like the current one is. You can still have open shelving or legs on the bottom if that's what you like.