Look at any map you can find. The lake has to be assumed to be inside Malawi. Tz starts from a position of weakness. Malawi does not even need to respond to the noises; they have been very accommodating to this point. They should just do whatever they choose to do with their lake.
Thats a very shallow observation.
The heligoland Treaty did not focus on Lake Malawi, it was a "Temporary" agreement mainly to define the Northern border between German East Africa and British East Africa's "Sphere of Influence". Spheres of influence are lands, not interterritorial waters, unless there's an island in those waters, then the dermacation line will be formed. That's why british and German soldiers marched up to 1 degree line on lake victoria, so that German East Africa can have Ukerewe, the same was done with the mocambican border because they had islands in between.
Since we don't have habitable islands between us, Lake Nyasa was considered to have a status of "International" waters and German and British had their gunboats roving arround up to 1914, when Hedwig Von Hossmann was blown as the first Naval engagement of the first world war.
Malawi will never own the whole Lake, until all Tanzanians are dead. The Heligoland treaty was never meant to give Lake Nyasa to Malawians, and if you peruse The British House of commons Hansard dated March 24, 1959 there you will find how the lords back in England view Nyasaland as being the country laying on the belt "along" the Western shore of Lake Nyasa.
Malawi will always be a "Shame" to mother Africa,
A nation full of cry babies, owning Islands on Mocambican waters (like your fellow B!#@es Kenya on Migingo), and made believe by your God, Kamuzu that you own the Lake and part of our land.
Please do us a favor, shut the hell up and keep tending to our gardens, and babysitting our kids, we appreciate having maids who speak English but please don't start nothing, It won't be nothing.