Good questions and observations. Regarding looking through or over the windscreen, this is what I've found. No matter how fast I go, at some point, I'll slow down and the rain will land directly on me. Then I'm looking through rain drops on my visor and
rain drops on my windscreen. If one is bad, two didn't make it any better.
Some windscreens are impossible to look through in rain. The Givi AIRFLOW is one. The upper adjustable portion gets beaded over and it is only slightly better than looking through a bathroom window. I've had times when I've been riding in the early morning hours and the road dipped in a valley where there was some fog...and the OEM windscreen and mirrors both fogged over in seconds. You would think the wind pressure would clear them off, but it didn't. All those are reasons to get a windscreen you can look over, because while you may chose to not ride when rain is in the forecast, you can't predict when you'll encounter that fogging condition.
As I wrote before, RainX is your friend. I've never needed a wiper attached to my glove finger yet when using it, and I've ridden a little in the rain these past few years.
If your visor fogs up, that's normal. There are lots of products you can use on the inside of the visor to help. One I've used in the past, is liquid dishwashing soap. Smear it on the inside, let it dry, and wipe it off. The soap contains glycerin to cut grease, and is the same active chemical in most anti-fog products...and it doesn't cost much.
When I first began riding again, I read various articles about riding in the center of the road. Some said not to under any circumstances. Another had a good point. The only place you really encounter an accumulation of oil is where cars stop. So avoid the center when you come to stop signs and lights. As you've pointed out, when it rains, the water pools up in the tire tracks. When that happens, I'll usually move to the center. You can also find the water isn't pooled up, if you're following another vehicle. At times like that, I'll move into the tire track...but it all depends on conditions.
I'll move around from one place in the lane, to another. It all depends on the conditions. Often, I'm moving just to be seen. It seems "wrong", but I'll ride in the right tire track on I-5's HOV lane. I'm only feet away from the cars I'm going by...but they can see me coming in their rear view mirror. If I'm over to the left tire track, and especially if following a car, I'm invisible and where I'm riding appears to be an empty space for a car to pull into. It must work because I've only hit my brakes hard about 7 times in over 55,000 miles of mostly freeway travel. (I also wear a hi-vis yellow safety vest that helps make me more visible.)
I found one thing though that keeps me from riding in the center of the lane. In the first year of owning my Burgman, I got 3-4 punctures. I haven't had any since that first tire. The difference is that I usually don't ride in the center. The cages out there do us all a service by picking up the metal debris on the road in the tire tracks. When you ride in the center, they haven't had the opportunity to clean it off for you, so you pick those nails, screws, cotter pins, etc. in your tire.