Your lawyers are ill-informed.
No, they most certainly are not.
Your lawyers are ill-informed. ICAO cannot impose anything on anybody. In certain fields, such as noise and NOx, it sets internationally-agreed standards which are non-negotiable (unless you opt out). But it is not a legislative body.
Fully agree - it is not a legislative body. But under Doc.3700 and Resolution A37-19 as it pertains to Annex 16 - for which there will be a volume III relating to CO2 emissions - signatories are committed to import into their national law ICAO recommendations. HOW it is imported is up to the individual nation concerned. Working Paper C-WP/13790, submitted jointly by Argentina, Brazil, Burkina Faso, Cameroon, China, Colombia, Cuba, Egypt, Guatemala, India, Japan, Malaysia, Mexico, Morocco, Nigeria, Paraguay, Peru, Republic of Korea, Russian Federation, Saudi Arabia, Singapore, South Africa, Swaziland, Uganda, the UAE and the United States attempted to force a recommendation in this direction - see actions d, e and f.
Let me have their name and I’ll invite them round for an expert briefing to put them straight on ETS, ICAO and whatever else they might be a bit shaky on.
Please accept my apologies but I won't take you up on that offer. I'm sure your advice would be relatively cheap in comparison but, well, you get what you pay for.