During ww2 the japanese offensive u-go sought to capture northern burma, forcing the allies back into india. The japanese command seemed to have some weird reasons to invade india, so if the japanese won a decisive victory at imphal, and broke into india what would happen? I honestly doubt the japanese could even take india let alone, Bengal. Due to massive hostile population, a population which would likely resist and with the appearance of putting india at risk britain focus their more also to stop the indians exploding with out rage of british failure to protect india. So what to the japanese if they broke into india?