Google will follow links from anywhere to anywhere. You don't have to have each and every page linked from the home page (and on medium and large sites, that would be impossible to do efficiently anyway). What you do need to do is to make sure that there is a route whereby the spider can follow a series of links that take them from the home page eventually to the destination page. As long as there are inbound links from a page that [I]is[/i] indexed, your page will get spidered, but it might take longer. Assuming the site map is indexed, that will be a valid route.
Google will periodically spider each page that it has in its index. If a page is still there, it will remain in the index. If a page has lost a lot of its inbound links, it will probably drop in the rankings, but as long as it remains live, it will be indexed.