Yes, travel via Weymouth is allowed for all journeys that pass though Dorchester West and South as both these stations plus Upwey and Weymouth are part of the Weymouth Group.
GROUP STATIONS
Some stations are grouped together to improve interchange between trains by offering customers access to a wider choice of train services and station facilities. A customer may travel via any station in such a group, including doubling back, provided that the group is on one of the permitted routes between their origin and destination stations. This extended availability is for interchange purposes only and does not apply where the origin or destination stations are part of a group.
ReWind, I note that a Off peak return (SVR) Yate to Bournemouth route either Yeovil or Salisbury is ^31.70. It may save you a few ^ to get a SVR from Yate to Weymouth route Yeovil for ^23.30 and then excess the return leg to route Salisbury for ^4.70 and break your journey at Bournemouth saving ^3.70. Just make sure that they don't take your ticket off you when you exit at Bournemouth, those
SWT▸ RPI▸ 's can be quite, umm, keen!
Cheers,
TLM