I'm shocked how the CVT is even running now. ATF is like poison for CVT Transmission.
Loosening just a nut for drain in this scenario is not recommended. You should have get the chamber opened. I feel your mechanic is not skillful enough to handle CVT transmissions. 3L amount is less and I'm sure there is still 1L or 1.5L inside transmission.
Green2 is a cvt code which is same in my swift cvt. Liqui Moly Top Tec 1400 is recommended for Green2 CVTs, you can check from LM website. It costs around 1800 for 1L but worth every penny.