Originally Posted by pdhenry
I'm suspecting that it will price out cheaper to book it all together since most one-way flights within the US are expensive (except the low-cost carriers; don't know whether they will help on this itin).
It may or may not. Most cheap LON-US tickets do not allow for a stop-over. Having a stop-over tends to bump you to a substantially more expensive ticket, even more so when you have two stopovers as in the OP's case.
ORD-MSP oneway starts at GBP88+tax on NW MSP-SEA at GBP38+tax on Sun country.
Try both: 1) as a circle ticket and 2) as an open-jaw+ buying the internal segments separately
and see what prices best.
But, to be frank, this is precisely the kind of situation where a good travel agent can help you and save you lots of time and money.