Ancestors in hiding

I'm looking for info on my elusive Great Grandparents Walter William & Elizabeth BUSST.
I can find some birth & death references but nothing in the middle from census etc.
Walter was born 1857 in Wolverhampton & died 1934 in Salford Gt Manchester (can find B & D on BDM index)
Elizabeth was born Elizabeth WILLIAMS in Wales (can only find D in BDM index) Williams, born in Wales

There were three children John JACKSON, Walter BUSST & Aileen BUSST (my Grandmother).
Questions I need to find answers for are:
1) Why no census records?
2) Why no marriage record?
3) Why has John got a diffrent surname to the other children?
4) Also after her death my grandfather said my grandmother (Aileen Busst) grew up in a children's home, how do I research this?
Any assistance or pointers as to where to look next would be greatly appreciated. Also neither of them are listed on the Manchester Council online burial/cremation records but the Manchester link is confirmed on my grandmothers birth certificate. Unfortunately my grandmother would not talk about her family & my grandfather mostly kept to this after her death.
Thanks
Andy