GitHub's api permissions aren't very granular. We can choose between all public repos, or all public and private repos. A lot of our users have their site in a private repo as it is for an internal wiki or simply a site they are still developing. Giving the user the ability to choose between all public and all public and private complicates the flow, especially if you chose all public when you started and then later want to access a private repo. This is something that we will take into consideration as the the product grows though.
Your posts should be listed with the most recent first. This seems to be pretty standard across content management systems. However, we will have sorting out into production soon.
I'm not 100% certain what you mean with regards to frontmatter. Right now, when you create a new page/post the fields that show up are determined by the frontmatter defaults (in Jekyll) or the archetype (in Hugo). The type of data that a field accepts is derived from the content in existing fields. We are working on giving the user the ability to change the content type of a given field and to add additional frontmatter fields.
The fields are derived from the frontmatter in a content file. If the content file has a date field in frontmatter, you will see a date field. I realize that Jekyll incorporates the date into the name of the content file so you may not have a reason to add a date field to frontmatter. I'll add that to my todo list.