Reason #1 as per OP was : "first time traveller" ...how is that correct for 28 kg vs 46 kg?
by "travelling for the first time" probably meant "this is first segment of itinerary" - so ticket is ex-India
Regardless, you are correct... None of the reasons provided make sense, although the policy they quoted is correct.
And air India communication never stated 46 kg. so sue em for what? For not recovering on the 8 kg they left out?
Have you seen the baggage allowance page? It is one of the most disorganized pages on the AI website.. It is not unreasonable to be confused about what the allowance is - there is no mention of India-LHR-India allowance anywhere on the page, and the itinerary showed blank for baggage allowance...
Now, if OP had called, s/he would have gotten the correct answer. But is it reasonable to expect OP to call when they have gotten what is seemingly the answer to their question on the website?
I'm not sure whether this event could be considered grounds to sue - I'm not too familiar with Indian consumer law. However, it certainly is a possibility which OP can explore.
Sometimes, it's easy to forget that travel is a complex and stressful thing for those who don't fly often - it's really not that hard to be confused by the page, and it's AI's responsibility to communicate the baggage allowance in a clear manner.