Honestly, there does seem to have been a fair bit of fear-mongering within this thread!
In my own humble opinion, a child seat compatible with a lap belt should probably be best secured by a lap belt. Properly tightened, a lap belt will be MUCH more secure holding the seat than the inertia-reel belts fitted to modern cars.
It is true that the rear seat back in a 1957 Minor is designed to tip forward but this can quite easily be secured so that it does not - if that is deemed a problem.
All the information within the workshop manual relates to the fitting of FRONT seatbelts. Most Morris Minor owners with children (and there are many) will be more interested in REAR seatbelt fitting. There is no factory approval for this simply because they hadn't been considered back then. It doesn't mean that rear belts cannot be perfectly satisfactorily fitted. My own children are grown up and drive their own cars now but we happily sourced and fitted rear belts in our Minor and carried them all over Great Britain and on foreign holidays too and would do so again. We are all aware that the huge 4x4s our streets are now littered with provide more occupant safety but it is up to each of us to assess risk and if we want to drive our children in a Morris Minor then we should be free to do so, fit seatbelts the best way we can and then use them

.
As I am anonymous on here I can tell you without fear of Social Services knocking on our door, that we also took our children camping, swimming in the sea and allowed them to climb on trees, rocks, bicycles, donkeys and to eat food cooked by their Mother who didn't always abide by "best before" dates

. What FUN we all had!