1. total coincidence.

  2. avast free is an anti-virus only it isn’t a suite with a firewall. So you need the windows firewall at the very least and better still a third party firewall that includes outbound protection.

  3. you actually don’t download the emails in text format as you have no control over the format that the originating email is sent. Most are multi-part/format emails, in that they contain both a text format part and an html format part.

Then depending on how you have your email client set to view your email it will display that format (if the sent email has that format in it).

Avast does not disable the security centre, it has no need or function to do that.

What changed the settings is not known and these aren't that much of an issue as this is the reporting/notifying of either your firewall, anti-virus or windows updates being switched off/disabled. In your case having switched off the windows firewall the security centre would normally have displayed a notification that it was off/not running.

Since there are no other indications of malware from avast, SAS or MBAM, it is not easy to say what changed these settings.