Shucks! That is major.
Was that an authorised dealer?
I have seen a few cases on this same thread (especially one Satya Electronics in Pondy?) and have also read all the replies.
Taking this up with the official LG India people is the best route to go.
What the vendor did was obviously devious, but I personally wouldn't pursue the legal route - on an average, consumer cases will take about 4 years for resolution.
If I were you, my steps would be:
1. First of all, a higher model replacement with full free installation. In your case, that would be a 2020 GX, or the 2021 C1 or G1. (Already having seen your set up, ask those cheats to give you a replacement G1 at no cost)
2. Ask for LG to take punitive action against the vendor and show proof to you - of course, this will bring the vendor into the picture and they will try to apologise, grovel, and maska maro you. How you deal with them will be up to you and your judgment.
3. All said, a clear proof of a strong reprimand to the vendor and a G1 as a replacement with full installation and calibration is the proper penance I personally would have demanded with LG.
It is, of course, sad you had to go through this, but a strong deterrent to the vendor without having their dealership cancelled and a satisfactory replacement for you would be the best solution, IMO.
PS: With the number of such experiences I am seeing online, I have a feeling that vendors are just shipping out demo pieces left right and centre and hoping they will get caught in at best one or two out of 10 cases. Very poor ethics and professionalism.