I'm also a Kindle user, but not distressed as much by the deficiencies that you have noted.
1 - My Kindle stays in its cover at all times. I fold back the cover to hold the Kindle in my left hand. Keeping the cover on while reading also makes the entire package thicker. I'm right-handed and just cannot be happy having my "good" hand tied up holding a book or the Kindle. I multitask, so having my right hand free to pick up my fork or a Diet Coke means that I'm going to hold the Kindle in my left hand.
2 - Some people have used the elastic on the cover to give them an easy way to hold the Kindle. Fold back the cover & loop the elastic around the front cover. Now insert your right hand either between the two covers or simply under the elastic and you can "hold" the Kindle needing no grip strength at all.
3 - My first reaction to the eInk screen was an impression that there wasn't enough contrast between the characters (ink) and background (paper). As I read, I realize that there's plenty of contrast -- but the Kindle's white frame emphasizes the fact that the background of the screen isn't white.
4 - I keep the Kindle on font size 2 in most well-lit situations. If I'm in a low-light environment, I enlarge the font to 3 or 4. Or, I use a flashlight. I don't have a book light that I like using with the Kindle and haven't really pursued it yet.
5 - I'm surprised that you are getting poor response with the Whispernet. Are you sure that it's the network? I wonder if the problem might be a slow rendering of web pages by the very primitive browser in the Kindle. Maybe you're expecting too much of the Kindle as an Internet appliance. I live in what I'd term a remote area 55 miles north of Dallas and about 25 miles south of the Oklahoma border. I get excellent response from the Amazon store, though the pictures (cover images) load slowly. After purchasing the book, it's completely downloaded in {usually} under a minute.
For my needs, the Kindle is an ideal device for recreational reading. I wish that there were more books available at lower prices, but I couldn't read any faster even if they were available. I have 'way too many books in my backlog; both paper and electronic. (This is what my TBR stack looked like on 17 February when I received my Kindle:
http://www.elsi123.net/~elsi/pictures/Image1.jpg The stacks on the floor are taller now and I've flowed into a fourth stack.) I only have about 80 "books" loaded onto my Kindle. About 10 of those aren't books at all, but text files that I have transferred to the Kindle for reference.
I do have a reading plan which encompasses both paper and electronic books. I read on average 10-12 books every month. It varies depending on how much travel I do.
I'm sorry the Kindle isn't the device that you need. If you find one that works better for you, please post a message and let everyone know what you settled upon. Fortunately for you, Amazon's 30-day money-back guarantee gave you the opportunity to try the Kindle in your own environment; which I think is better than simply handling a device in the store.