Which realities fall within the definition of a body governed by public law? Can a national association of municipalities or a regional environmental protection Agency being a Lead Partner, for example?

As indicated in the Programme Manual, section I.B.ii. bodies governed by public law are defined in Article 2(1) of Directive 2014/24/EU on public procurement Public equivalent bodies are:
• established for the specific purpose of meeting needs in the general interest, not having an industrial or commercial character;
• have a legal personality;
• financed, for the most part, by the State, regional or local authorities, or by other bodies governed by public law; or are subject to management supervision by those authorities or bodies; or have an administrative, managerial or supervisory board, more than half of whose members are appointed by the State, regional or local authorities, or by other bodies governed by public law.

Furthermore, each Partner State is responsible for confirming the legal status of partners located on its territory. Therefore, in case of doubt about the status, you should contact the relevant National Authority. The contact list is available on the website of the Programme.