Mike, you have a lovely plant there. If you prefer you can bury the graft site but it is not necessary.
A Dwarf Singapore makes an excellent pot plant, but there are now quite a few compact varieties that might also do well for you. Here is a 'Thailand Red' - I have this variety and at 8 years old it is only 5 ft tall and wide.
(By the way - the picture is watermarked for Plumeria2u.co.uk but I supplied it to them originally... so it is mine to use
)
