Starting from c there are two possible places to aim for: Swinburne and Harrismith. There is also the option of breaking the journey after about 33 km on Collings Pass, at the Waterfall farm B&B: it's up to you to decide which distance suits you.
Starting from Mawelawela B&B, you back-pedal about 4 km S, past Gerta's farm stall (where you'll want to stock up on witels if you plan to overnight at Waterfall cottage), to where the gravel P263 crosses the tarred N11. Turn right here because this is the Collings Pass road. After about 7 km of steady ascending through rural KZN (Matiwane, named after a Zulu warlord around the time of Shaka) the road branches to the right, then you cross the Cwembe stream, a tributary of the Sundays River that flows into the Thukela.
You continue ascending up through Craigsforth scattered settlement. The road dips and tops as it follows the general course of the Sundays River. At about 30 km you'll see the Waterfall farm sign on the right (for ham sandwiches, turn in here, but you'll have to bring your own), and further upstream until about 40 km from the tarred road (1 140 m) past a large dam on your right (and circular irrigation field on your left). From there the route follows a windy-windy track along the Mhlwane tributary till you reach the Free State border on the watershed at around 1 800 m altitude.
All the while the road has been getting narrower, less frequently used, and deteriorating (we would say improving), from a normal district gravel road to a 4x4 track. You ride around Nelson's Kop, one of the impressive sandstone castles, and then reach a crossroads and a decision: keep left to Swinburne, the Catfish and Caterpillar Cookhouse and then mountain-bikeering around Sterkfontein Dam to Phuthaditjhaba, or right to Harrismith (keep left then swing left again onto the tarred R34 to town), Kestell/Golden Gate and then Phuthaditjhaba.
Advice for the young at heart: go the Swinburne way for the adventure tour (just across the N3 - take care!), or Harrismith for the scenic tour. Either way, you are riding through some of the most sublime scenery the region has to offer, on roads made for mountain bikes, so it's a win either way.