R & B,
I'll bet there are a lot of UK pilots who dream of having your dilema!
Sounds to me like option B, the Dash 8 job near the US offers you the best compromise between building your career and fulfilling your family obligations. If there was some way to get your fiancee out to the UK to take the boeing job, that would deffinitely be the best. If that is not an option, the Dash 8 job sounds pretty good to me, you get to build valuable turbine time in a crew cockpit enviroment flying for (I presume) a scheduled part 121 operation.
If you later decide to switch to a US operation you will find that your Turbine experience will give you a lot more options and probably allow you to upgrade to Captain a lot sooner.