Might be worthing flying into Swazi, depending on the day there is an 06:35 Airlink flight from JNB to SHO. I did overland from Maputo to Mbabane via local bus/chapa about 10 years ago. It took most of the day so I don't fancy your chances of doing it in a morning before a 12:30 flight.

I've been a couple of times and it's not exactly long on food/culture/architecture. Nice people, very chilled way of life but it's the scenery and wildlife that are the main draws. If I were you, I'd fly into SHO, spend the rest of the day around the airport which is the Moz side of the country and then overland the following morning. Maputo has a lot more going on.

I've seen the Swazi shuttles and others (Citybug etc) on the N4 heading East. Bad fast driving and good enough roads that allow bad fast driving make it a fairly scary experience.