OP: I hope that today goes well and your family get on the LGW-MRU later.

Reading your story gives me - and I suspect plenty of others - collywobbles, as I've done similar things for family hols pre-Covid. Twice using BA companion vouchers to India (Delhi and Chennai) then onward on separate tickets (Bhutan and Maldives). Both times the Indians, contacted via the airport website, emailed to confirm no Indian visa required, just find someone near immigration on arrival and tell them what you're doing. I even asked about checked luggage and they, in the email, said not to worry, they understood BA wouldn't through check but they'd collect it and sort it out. Which is indeed what happened. Clearly, we were lucky at T5 to be allowed to board without Indian visas.