In a page with subtabs, can I not have a main entry page and only have the subpages?
Unfortunately, no, that's the way it works in the system. We don't just automatically send folks to the first sub-page in the list because that initial tab could have content - you have that choice. Guide authors do often put content on that page in addition to the subpages they create.
Some users when seeing a submenu will honestly assume that they have to choose one of the subpages and won't necessarily click that main tab.
And if they do happen to click on the tab, the system is coded to just automatically generate a list of sublinks (if there isn't any other content on the page) so your users will know that they have to choose one of those other pages to get to any content in that section. This is just like the way a lot of other websites work, though those other websites may have flashier landing pages for the main sections of their sites than just a list of links. ;)