Getting Back to Darwin from Borroloola

Hi, I am looking for some thoughts on how to get a German friend who we will be picking up in Darwin and travelling with us back from Borroloola or close by back to Darwin for his flight home.
We will be taking him to Litchfield NP, Mary River NP, Kakadu, Edith Falls, Katherine, Mataranka and then on to Daly River's Pub for an Aussie night.
Then we head across to Butterfly Springs/Lorella Springs/Lost City area.
We then go onto Adels Grove and karumba but don't have the time to get to Townsville etc on the east coast for conventional buses or planes before his plane departure
.
Any ideas???????

Thanks in anticipation of a brilliant answer ( and this does not hopefully include me driving back to the Stuart H'Way) -even as this might be the only answer
Neil