In case anyone is interested, I got a quick response from Comic Con re: above questions:
"Professional registration is not required to apply. The Small Press Pavilion is strictly for small independent publishers, and every application has to go through a jury selection process. For SDCC 2018, along with your application, you are required to send in a current sample of your own self-published publication based on the following guidelines:
-A physical copy of the sample must be sent in to:
CCI
PO Box 128458
San Diego, CA 92112
Attn: Small Press
-The sample cannot be published by any other publisher (DC, Marvel, Image, etc) it must be your own independent company.
-The sample must be postmarked on or before the posted deadline. The application for Comic-Con 2018 will be available on our website on July 24, 2017. Details for the deadline will be on the application. Any sample postmarked after the deadline will not be accepted.
-The sample must have a storyline with sequential art. No art prints, sketchbooks, etc. will be accepted.
-*Very Important* The sample must have a copyright and/or publish date of 2017 or 2018 clearly printed on the sample sent in. No other copyright/publish date will qualify
-There are no exceptions to these guidelines
Once the Small Press Board has review all the samples and made their decision of who gets selected, notices will be sent out and if selected, payment will be required then. You get 2 exhibitor badges and can purchase up to 2 more if needed."