Of course double check, but that should be fine. The SQ only RTW allows for surface backtracking within Europe not sure about the SQ/UA one.

I've done EWR-AMS (surface) LHR - SIN on this fare (SQ only). (Back when the EWR-AMS flight was on).


EDIT: Just read the fare rules for the SQ/UA fare and it doesn't appear the Europe backtrack applies for this one.
(You can find the fare rules by doing an around the world fare search at travel.com.au - my link to the fare rules doesn't appear to work)