The documentation is indeed a bit redundant here, and not really in sync with the more detailed installation guide where this is mentioned.
It's fine, CentOS maintains older versions of packages they ship as long as the CentOS version is still supported. I'm not familiar enough with CentOS to say whether upgrading python is a good idea (it's generally a bit of a hassle on debian/ubuntu), but as long as you manage to get your certificates, there shouldn't be any issues in the future (unless letsencrypt drops 2.6 support completely).
That's okay, the email will only be used for renewal reminders (and in the future possibly account recovery if you lose the private key).