Currently SIA does not code share as such over IAH. For example, it offers Singapore to Austin and also Nashville, both served by United Express from IAH, over the Pacific and on from the US west coast on Virgin America. This is possibly because the SIN-DME-IAH flight is seen as a point to point operation with better timings and connections over the Pacific and few using the DME-IAH sector for onward connections.
The United/United Express operation at IAH is huge and has a massive "push" from around 16.30 each day ideal for connections. The way I look at it, the new service will see less than 30% travelling through to IAH from SIN - the trans Pacific timings and connections are much better for much of the US, and for the North East the flight over AMS to JFK gives a much earlier arrival. There is a reasonable amount of freight between SIN and IAH and vice versa, Manchester has oil related freight for Texas and, having decided to give up on Moscow, breaking the flight at MAN offers a win win situation - it offers three distinct services:
SIN-MAN and return
MAN-IAH and return
SIN-IAH and return
Manchester will offer a far greater revenue potential both east and westbound than Moscow ever did. As for connections at IAH the Star Alliance partnership is there to be exploited and, even without code shares, any travel agent/desk/website will point out the Star Alliance link to holders of the various Star Alliance frequent flyer cards, though my betting is on a developing codeshare scenario as demand grows.