However, the ticket was available, at no extra cost to me, so I took it. But is it valid?
Here it is, without a railcard,on the GWR site:
If offered and bought based on that offer, my understanding is that it's valid. I would take a printout / screen capture with me, as this fare is lower than a Maiden Newton to Weymouth fare, and it may not be an option that's well known to staff.
I did have put in "via Weymouth" to get these timing options as the system cheekily offered me rail journey plus walk across Dorchester as default suggestions - much quicker, but if I'm not very good on my pins, that might not be a valid option.
With regards "doubling back" - yes, I'm bit surprised as you could / should change at Upwey. However, looking at disabled access there the National Rail site specifically recommends carrying on to Weymouth
Staff help available: No
Ramp for train access: No
Step free access coverage: No
Step free access note
Platform 1 (for trains to Poole and London) has level access.
Access to platform2 (for trains to Weymouth) is via a footbridge with steps.
** NOT SUITABLE FOR CHANGING TRAINS - PLEASE USE WEYMOUTH **
Wheelchairs available: No
So there you go - if anyone's not walking from Dorchester West to Dorchester South - which is not very far - it's probably because they have limited mobility in the first place!