It likely would have been an entirely different movie altogether, Ramonda's death was to motivate Shuri and T'Challa would not need to be motivated by her death to spring into action. I doubt she would have, maybe been injured, but not killed, because anything less than Namors death would be THE SECOND villain T'Challa spared after killing a parent. If he were alive still, it was barely acceptable than Zemo survived, Namor? Nah he would need to be decapitated for that isht or he would have a forever W over T'Challa