By default I use Facing page layout for all my documents. However, opening documents that have only 1 page in Facing page layout makes the page to be displayed in left-of-center position (needed for Facing page layout).
Auto switch from default Facing (if used by the user) to Single Page page layout if a document has only 1 page would be a sensible feature to have.
Foxit pdf software does this be default. If a user uses Facing page layout and opens a document containing only 1 page, page layout is automatically switched to Single Page page layout and all other page layout options (Single page continuous, Facing, Facing continuous) are greyed out (disabled) as there is only 1 page in the document.
Document has 2 or more pages (Facing page layout is used if a user has selected it in preferences as the default page layout option and all other page layout options are available) ↓
Document has only 1 page (Single Page page layout is automatically activated and all other page layout options are greyed out) ↑