Hi,
I have taken this route before (about 3 years ago) and I would recommend it.
I left Inhambane and went to Vilanculos where I spent a few days by the beach. It's a pretty spot and a good place to visit the islands off the coast, the trip from Inhambane took the best part of a day at the time but then the roads have been tarmaced recently which would decrease the travel time.
After that headed to Chimoio although if I remeber correctly had to change buses at Inchope which is not far from from Chimoio (note the bus from Vilanculos to Chimoio was 4-5am in the morning and took all day). There is a large hill near Chimoio shaped like a head which is easy to climb and gives beautiful views of the city and surrounding area. From Chimoio to Tete the bus also leaves very early and it would be advisable to scout out where it leaves from the day before you expect to travel.
In Tete you can get a bus which will bring you to the border (Zomba). The ride from the border doesn't take long but you would be advised to pick up some Malawian currency in a currency bureau in Chimoio - There are blackmarketeers changing currency at the border however it is illegal and can lead to problems with the authorities (especailly if they are in league with the blackmarketeer!)
All in all if you were to take a bus from Inhambane to Blantyre I'd say you would be looking at about 3 days travel by local transport (at least if the roads are anything like they were). While a flight won't take long it could cost you a small fortune due to the BASA's in place in Mozambique.
I hope this is in someway helpful, whatever you do have a great trip, Inhambane is a lovely spot as is Praia de Tofo nearby!
P.S. Stayed at the Pink Papaya in Chimoio which is run by a Zimbabwean woman. I tink she also runs a farm about 100km away where you can learn about Zimbabwe and its recent history. While it sounded inteesting I didn't make it due to time constraints at the time.