
Hi There
Good to see you have done your research, but herewith some comments and recommendations:
The entire road from Bergville right up to Thendele is tarred and should be no problem for your hired car. There is parking very close to Thendele, right at the start of the Tugela Gorge Trail.
I would strongly recommend against attempting both routes in one day. The Tugela Gorge will give you 14km and the Summit walk will be a further 20-25Km making close on 40Km for the day, including some significant altitude gains from about 2000m to 3000m. One would then have to camp on the summit overnight, or return adding yet a further 20-25Km, which to my mind would be excessive and dangerous.
I would hesitate to call the Tugela Gorge route flat. Rather it is at a sustained gradient uphill most of the way which can prove strenuous depending on your fitness levels. I would rather work on two to three hours each way, especially if you want to take in the views, take photographs and not exhaust yourself. I would suggest 4 to 6 hours for the round trip bearing this in mind.
You can then drive back down to Mahai and park / camp there. The hike from Mahai to Mont-Aux-Source is a good 20-25Km one way. The first half of the route is again at a sustained but pleasant gradient, but gets steeper as you approach Witsieshoek. Later on after that the path ascends past Sentinel Peak and can be steep, whilst the altitude also becomes tiring. As mentioned above you would have to choose to camp overnight, or return making for a very very long day which I would strongly discourage. If you choose to camp overnight please read the security recommendations elsewhere on this forum.
As a recommendation I would suggest you set aside one day for the Tugela Gorge route and then relax the rest of the day at Mahai Campsite. Thereafter if you are equipped for the overnight route (and not alone) you can do the long hike to the summit, overnight and then return. Many, if not most, people would rather drive to the Sentinel Car Park and do the summit walk from there which makes for a decent one day's hike. The road to Sentinel is for the most part fine, but (its been a while since last there) deteriorates near the top. However I expect it would still be okay in a hired car
Finally whilst the RNNP hikes are generally very popular especially over weekends I would recommend taking extra care if you are hiking alone. I would certainly not recommend the Summit route to be done alone !