diverging

You may have to download the the data for the whole lake. Select the way you want to split. Under relations you should see Lake Алла-Аккаярви and to the right a download button, a delete button, and button with a square and a dot in the center that highlights the whole lake. Just press the download button and you should be able to split it.

What they are suggesting is certainly possible. A segment can be shared between two different relations.

The process would be: 1) Draw the line around the boundary of the forest, connecting the endpoints to points on the lake. 2) Split the way that is part of the lake at the two endpoints of that new line. 3) Select each way you want to be in the new multipolygon and create a new relation.

Doing this makes it so that there is a single way with two relations on it, instead of two ways (each with a single relation) that share a bunch of points.

if i delete dependencies from lake-coast (wich is multypolygon by itself)

You shouldn’t need to delete anything. I am not sure, but it seems like you are deleting the members of the relation, those definitely need to stay.