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The lakes and the bay are formed by the rift where the right chunk is diverging away from the main plate. I've put mountains on the land part of the rift a la Ethiopia's Great Rift Valley, but should there also be mountains along the edge of the lakes/bay? Do the mountains formed by a rift go on the part of the rift that's currently over land, or should they also be on the coasts of the part that's already split away?
For the land portion of the rift there would be a rift valley with volcanic ridges on either side.
For the portions in the water there will be an underwater ridge. If enough magma rises under the oceanic crust then a string of islands can appear. But this is very rare. The best example I can think of it Iceland, which sits directly on the Mid-Atlantic Ridge.
For the shorelines you will have older mountains that are no longer volcanically active.
The left shows a rift of a continental crust and the right is the rift of an oceanic crust.
This depends on what you mean by mountain.
Rift valleys form a smattering of small isolated volcanoes. They occur everywhere in the area and are mostly a side effect of the upwelling mantle magma that creates the spreading finding any crack or weak point.
There is also usually bit of flood basalt where the rift triple junction first formed which gets carried along by the diverging plates.
The typical "mountains" of rift valleys are not really mountains as we normally think of them. just the original height sides of the rift valley which seem tall by comparison to the valley. these are called "block mountains" See below
