Even if you get such expert opinion, they can just choose to ignore it. I could do it but I doubt it would be of any help, and they can always just question my formal qualifications, simply because such formal qualifications to write such statement simply do not officially exist.
Assuming that product specifications (brochure shown to you when you bought the unit) actually specify operation well below -15degC, I guess your best bet is
1) consumer protection authorities,
2) media,
3) social media,
or combination thereof. It's amazing how much a negative article in press, or a tweet, can do with companies who refuse to perform their legal obligations.
Fixing issues, by installing a completely new unit if that is what's needed, is normal business operation and covered by sales margin and installation costs. Besides, sales profit and install costs of air-to-air heatpumps in Finland have gone up so much you can actually buy 2-3 units installed, in most other countries. But this sure can't be seen in install quality or guarantees.