Um, do you really think that early Christians rebranded a festival for marketing purposes?
Yes of course they did.
Well you seem very sure but most of what you quoted is just theory, and there are other, contrary theories about the dating of Christmas. The date of December 25th was fixed around 250-300 AD but there is no evidence of the assimilation of pagan traditions into the Christian church at that time- that came a few centuries later. There is evidence to suggest that early Christians did not want their celebrations associated with pagan rites and actively distanced the birth of Christ from them. Christianity did not develop in isolation so bits of other cultures and beliefs were picked up as it spread across Europe, but the dates for Christmas were already set by then.
But of course if you want to believe your theory as if it were Gospel (excuse my pun)…
Back to Mothering Sunday:
Nope, wrong again. Mothering Sunday was revived around the time of WW1, made even more popular after WW2, and merged with the idea of "mothers day", which was an American secular card giving day, which is celebrated in May in America. And the name gradually changed.
But as you yourself say Spartacus, Mothering Sunday was revived after WW1, not Mother’s day. And Mothering Sunday became popular again after WW11, not Mother’s Day. It has now become interchangeable with Mother’s Day, to the point that the OP is having trouble finding cards with Mothering Sunday on them, so she is surely NBU.