I think subordination here should be taken as the ability of the imposition of one will upon another (thus why it is presupposed). A will that wills for the and in place of the subordinated will; that is what subordination seems to imply, an order of rank of wills. I believe that one has to take into account the use of the word will in political philosophy: for example, you can find the theme of sacrificing the will of the individual to the state for something (eg safety) in a number of writers. Even if you think that these words don't fit this use in the way you understand, I suppose that you still have to go with the writer's use. But imo, Engels' definition is in line with most of the anarchist literature of the time.
Now, about whether they are necessary, I would say that you have to take them as such by definition, since Engels seems to understand them as necessary to one another, as interdependent. But if you mean necessary in general, outside the text, then no, they don't have to be.
Of course not all contemporary anarchists would oppose them in all cases (still most seem to), but because I believe that ever text should be seen in its historical context, I think that it does a very good job in offering a critique of anarchism, or more correctly, an answer to the anarchist criticisms of Marxism that motivated Engels in writing that piece.
You are welcome 😘