I don't think you've got much legal grounds here. It's well established under intersystem treaties that organisations are allowed to keep the spoils of war including fixed and mobile installations. Tanner attacked an Azimuth subsidiary in an act of war, and regardless of his moral justification for doing so, he lost the war and lost the Musashi in the process. Azimuth are legally entitled to keep it. None of the superpowers or other major players are going to accept a revision to those treaties that also invalidates their previous years (and in some cases centuries) of conquest.
Additionally, the non-interference requirement applies to the current Aegis organisation; the Musashi was never the property of this organisation - indeed, the modern Aegis is at pains to point out that despite having the same personnel in key senior positions, it is not the same organisation as the one heavily criticised by Baumann.
Even if the non-interference requirement did apply, an interpretation that required all other organisations to surrender without a fight if Aegis attacks them would be the exact thing to generate massive distrust of Aegis "it's the shadowy conspiracy planning to unify all the superpowers and impose their own government" and not something to recommend.
3) provides funding to Aegis to makeup the shortfall resulting from previous funding interruption from the powers.
You've got rather more of a shortfall to make up than that. The superpowers aren't providing the new Aegis anything like as much funding as they provided the old one, even on a day-to-day basis.
- the Federation have allocated a junior Rear Admiral rather than the previous full Admiral as liaison, with a corresponding decrease in fleet and budgetary authority
- the Empire are handling it as an arms-length unofficial exercise funded entirely by Princess Duval's (albeit sizable) personal account, with substantial internal dissent (by Imperial standards, at least) that it's even gone that far
- the Alliance have no money anyway but are giving most of what they have for AX budgets to Sirius instead