i don't see how malawi can lay claim to an entire lake when it lies on a border region, even so tanzania will have a hard time due to some stupid colonial agreements and treaties. just look at the nile treaty where egypt stupidly claimed rights to the whole nile and its sources and declares it will go to war for it. Colonial era deals are usually a mess e.g look at Kenya and south sudan, illemi triangle now that oil has been discovered in the region south sudan wants it. Considering that the Kenyan map has had it from independence.
I don't know how Tz will deal with the issue but this is going to be a tricky affair, if it comes to war which i doubt it will Malawi will have no chance. i think thats why Banda has been trying to woo the west. recently didn't she just say that she will legalize homosexuality and fight for gay rights including Gay marriages. She seems to do everything she is told like a puppet. If things escalate, i think Tanzania will need to get a big brother from a powerful state to side with them on this issue, like Russia or china who have similar boundary disputes issues. If they don't, diplomatically Malawi will beat you. If the western bastards decide to finish you off they use selective sanctions just look at what they did to Iran and Zimbabwe.
As for the comments on SADC what do Kenyans care if our brothers from TZ are in SADC or not that is non of our concern. we are also in COMESA and Tz is not each country looks for ties that will suit them