My post might have been more helpful if I'd replied before you made your purchase, but I just saw it today and thought I'd post anyway.
I've been a B&N Nook reader for 4 years and due to myriad technical issues, not the least of which was some low-level "support" person somehow deleting my entire library, and an article that someone posted here about a proffered offer to buy B&N but not Nook or ebook division, I tried a Kindle Paperwhite. And I would never go back. Yes, what you say about the "walled garden" is correct, but I find that both the customer support and the ability to sync both page number as well as highlights/annotations EVERY TIME totally made me thrilled with my purchase. I'm a writer and I use highlighting/annotation a lot and now I have the ability to access all of it online, even for my sideloaded content. For me this was my overwhelmingly decisive factor.
As to your questions about the bright spots on the screen, let me offer this. I really like reading on eink screens and so my eyes were not seeking a bright background. So I keep the light at almost the lowest setting in bright light, with maybe enough to make the text "pop." And in low light I turn up the screen only enough to easily see the text. Personally, I don't like reading on the screen when the light is turned all the way up and this is the only time that the spots at the bottom are visible, at least to me. I also don't like reading in total darkness, which is why I rarely need to turn it up.
Your uses may be different, but I thought I'd offer this as you're getting use to your Paperwhite.
There are only two things that I don't like about it–that they decreased the onboard storage from 4 GB to 2 GB and that they still refuse to include a microSD slot. If I could add a microSC card, the Paperwhite would become my ONLY ereader from this point forward. I do use the Kindle app iPad app at times, if I need to carry my iPad with me for the day and they've done a very nice job with that as well.
Here is a site that publishes a list of free and cheap Kindle books every day. The deals are typically only good for that day though, and sometimes, if Amazon decides to change it during the day, the free or cheap pricing might not last the whole day. I've found some very interesting books through this link. Check in the afternoons though, as there is always a second round of books posted then.
www.pixelofink.com
Home this was helpful.