As it appears you are originating in the UK, the extra segment in Asia will cost you GBP75 for coach or GBP200 for business. You might want to consider buying a separate ticket BKK-SIN rather than buying an extra Oneworld segment. You should be able to pick up that ticket inexpensively in BKK.

Your NA segments look OK and you have used all six that are allowed. However, the way you have planned these segments, hkg-sfo-ord-dfw-sjd-dfw-lga-mia-lhr, will result in about 18,000 miles. You can hit the same locations, albeit in a different order, by using the routing, hkg-jfk-sfo-ord-mia-dfw-sjd-lax-lhr, and get about 4,000 additional miles.