how to drive from jo'burg to imfolozi

Or actually from Johannesburg Airport to Hlabisa which is a village just west of Hluhluwe-Infolozi on the R618.

There are 2 alternatives:
1) go via Durban (822 km) with the benefit of speed on the N3 and tar everywhere
2) go via Piet Retief (536 km) with a shorter distance, but slower diving & possibly gravel roads

We consider a stop over halfway anyway as we are traveling with a small child.

What are the road conditions when going via Piet Retief? What average speed is expected (50km, 80km)?

What scenery is better, more interesting (drakensberg & midlands vs. mpumalanga)?

Any good suggestions for the stop over (B&B, Farm, ...)?

Either way will be tar all the way.
I would go via Piet retief, and although I've never taken notice, I reckon your average speed would be 90-100kms. I haven't travelled it for about 2 years, but it wasn't bad at all, back then.

We normally leave Jo'burg early and stop for a Wimpy breakfast and fill up at the Engen 1 Stop in Piet Retief, so can't recommend a place to stay, but have a look here

On both routes, the scenery is flat farmlands, and not particularly interesting until you get to the escarpment. The N3 bypasses places like the Drakensburg, but travels through the midlands, which are rolling hills, whereas northern Kwa Zulu is more interesting for me, and a lot more rugged.

The route I would suggest, and the way I go, (altough it adds distance, and a toll) would be:
Leave Jo'burg International (now OR Tambo Airport), and take the N12 towards Witbank (you're heading more north, and on the route to the Kruger Park and Mocambique) but this way avoids the trucks and numerous gold mining towns you pass through on the N17. You join the N4 at Witbank, and roughly 35kms later, bypass Middelburg. At some stage here you turn right onto the N11, which will take you through Hendrina and on to Ermelo. From Ermelo, you'll rejoin the N17, which is now the N2, and Piet retief is roughly 100kms from here.

To get to Hlabisa, travel from Piet retief to Pongola (+-95kms) and then turn right onto the R66 for about 66kms to Nongoma,where you'll then turn left to Hlabisa, which is again about 66kms from here. I don't know these roads, but from maps, there is gravel dirt along both of these roads.

I did it in the opposite direction (St Lucia to Joburg central via Piet Retief) in one go. The roads were good all the way. I don't remember any gravel roads, but it wasn't the most thrilling drive. I think it took me about 6 hours.