You can buy the ticket at any railway station, telesales, rail appointed travel agent, or any rail ticket booking site (and possibly from a guard in advance; many guards would appreciate the commission).
It is unclear to me what you wish to achieve by trying to force the National Rail website to do this?
The National Rail website does not sell tickets. But in any case you will not get reservations returning by a different, slower, route by booking online, as you don't get the option to specify different 'via' points in different directions. Also note that there is no point trying to get reservations on trains out of Marylebone anyway as they do not offer seat reservations (as far as I know).
If you want to read the restriction code, to satisfy yourself that there is no restriction out of Marylebone in the evening, then to do this, make a journey enquiry, click Check Fares, select a ticket (e.g. Off Peak Return), then look in the blue box under this table titled 'Tickets' and you can then click on the ticket type in this box, this opens a page with generic information about that ticket type, scroll to the bottom of this page, and under 'Restrictions' there is a hyperlink that reads "view the complete list of 'Validity Codes' applicable to the ticket type that you have selected" clicking this hyperlink opens the full restriction text of that ticket in a PDF file. (A full list of all restrictions is available in PDF files for both
Advance and
Off Peak fares). In this case, a Stockport-London Terminals Off Peak Return has the restriction code 2C.