Hello,
We stayed at Barrowburn last summer - and can thoroughly recommend it - Ian and his wife are lovely and her evening meal and breakfast could feed an army.
As the 15 miles from Byrness was about at our limit we too were concerned that the descent should not be too arduous. We left the way just before Russell's Cairn, continuing to follow the fence at the point at which the way crosses it and following a clear track uphill. This brings you to the ridge and a gate. On the other side of the gate after a short steep descent, a wide grassy track can be seen going all the way down to the Rowhope Burn valley. This joins a road at a footbridge (marked Trows) and from there it's an easy road walk to Barrowburn. The route you've identified looks more direct and avoids the road, but in the dark and or bad weather I imagine the road would be safer.
The next morning due to atrocious weather, we wimped out as my sister who was due to help us celebrate a PW completion done in stages, was able to pick us up. So we still have 1 day to do.
Had we stayed, Ian would have taken us as far as he could back up to the ridge, which I believe is close to Uswayford - thus allowing you to meet your friends (but missing a section of the Way) or you could walk back to Russell's Cairn.
I'm sure Ian at Barrowburn would also be happy to give advice.
Hope this helps,
Briony