As you have already identified, your site's internal linking is important not only for making pages accessible to users and bots, but also for indicating their importance. The most 'important' pages on your site - for instance your key landing pages - will naturally be internally linked to the most, and search engines are able to understand that. Internal linking is also important to properly distribute 'link juice' amongst your pages, which directly increases their authority and equity.
The addition of an HTML sitemap may also help orphaned pages to be crawled. Though more for users than bots, HTML sitemaps help to un-orphan pages by linking to them and creating a point of access. It's also important to remember that user behaviour is something that search engines do take note of.
I have to admit that it seems slightly odd to orphan landing pages that you want to rank organically. I would say that usually landing pages that are orphaned - i.e. inaccessible from navigating the site - would be more for PPC or social or email marketing.
Hope that helps.