[DNSB]

1. You don't have to register with Kobo but I have seen no indications that Kobo is selling my email address in the last 13 years. Also note that Kobo does not check the email address for validity so, as an example, whoami@erewhon.net works.
   
2. Personally, I use the standard renderers (Kobo has 2 renderers, RMSDK which is Adobe's and allows for reading Adobe ADEPT DRMmed ePubs while the other is WebKit based and has better ePub3 support) for 98% of my reading. I also have KOReader and Plato installed but only use them for some PDF files. Installing is easy and does not require any jailbreaking/rooting/other hoop jumping. A matter of personal choice.
   
3. I use the Kobo covers mainly since they were the ones available as I tend to purchase a new Kobo ereader when it's on pre-order. I know people who have used other cases and while they have had a few issues such as mis-aligned sleep cover magnets, overall, they are decent quality.