Tanzania to Madagascar and back

Hi guys!

My hypothetical route is:
1) Alone Fly to Tanzania:dar es Salaam - Zanzibar - Madagascar - Dar es Salaam and perhaps some of northern tanzania (alone)
2) then joined by friend Dar es Salaam - malawi - Mozambique - Zambia & train back to dar es Salaam leaving my friend there the work 8 weeks before getting same train back to Dar es Salaam.

My main problem is that I cant find much info about the cargo boats to Madagascar and read they were dangerous and not running in July so didnt want to risk getting stuck there! could go mid may for 5/6 weeks then head back and do zanzibar and northern tanzania before my friend flys in as she is most concerned about mozambique and malawi......
If any one could help me get any idea of the cost and how best to get a cargo boat id really appreciate it as I dont want to book up any volunatary work on madagascar and find im stuck in tanzania!or that i get totally ripped off or mugged or something when im on my own!??

thanks crys xx