One area I've not known where to probe is Catholic records. I can't quite work out when/which parts of family were/became Catholic but as my grandmother Nell was brought up Catholic (plus my father, our family), I suspect Ellen Hart was Catholic too. But some of family seem to have C of E baptism, marriage records. Don't know if there are any RC records.
Catholic Family History Society for guidance on researching R.C. ancestors.
https://catholicfhs.wordpress.comOr do internet search for Catholic Family History Society. It has several sites. There's also a North-West branch of Catholic History Society.
Lancashire Archives
https://www.lancashire.gov.uk/libraries-and-archives/archives-and-record-officeSee: Our Collections . Church Registers Guide.
"Lancashire Archives is responsible for the archives of the Roman Catholic dioceses of Lancaster, Liverpool and Salford although many R.C. registers are still kept in their respective parishes."
There is a printed list of parishes in alphabetical order of place. 5 Catholic churches are listed for central Bolton. Lancashire Archives holds some registers for all 5. These are followed by 10 churches in suburbs of Bolton. LA has some registers for 7 of these. Next comes Bolton-Le-Sands. This is a different place much further north in Lancashire. (The full name of Bolton is Bolton-Le-Moors.) I don't know if this list is up-to-date. You can check current holdings for an individual parish in the online catalogue LANCAT. Bolton is in Salford Diocese.
GENUKI pages for Bolton
https://www.genuki.org.uk/big/eng/LAN/Bolton8 Catholic churches are listed, 5 existed in 19th or early 20th century. Page for each church has dates of existence and information on where registers and register transcripts are. GENUKI is a volunteer site. Information may not be up-to-date. Each church page has a map. If you know where your family lived at time of birth or which sub-district birth was registered in you could make a shortlist of churches. However a baby might have been born at a relative's home and baptised at the nearest church.
GENUKI has a facility to search for churches within a selected radius.
Lancashire Online Parish Clerks' website also lists churches in each place and has a brief history of some. LANOPC started adding Catholic registers a few years ago.
Around a century ago the Catholic Record Society printed and published transcriptions of many historic registers. The latest I've found in the journals for any of my families were from 1830s. Some of these have been digitised and indexed. The Genealogist and other FH websites have some. Many are on The Internet Archive.
NB Catholic Church in England has recently imposed a closure order of 110 years on sacramental records. This includes baptism registers but not funeral/burial registers. I think it's 70 years for marriages. A few baptisms up to 100 years previously had been put online before the decision was made to increase the interval.
There are a few people on RootsChat who will do look-ups at Lancashire Archives as long as you are specific about the information you need and the sources which might contain it. The Lancashire Archives look-up request thread is on the Lancashire board.
You mentioned that you're not sure when your grandmother's family became Catholic. This isn't unusual. Marriages pre 1837 had to be C. of E. to be legal. Some Catholics, especially poorer ones married in C. of E. after that date because it was cheaper. A wedding in a Catholic church required presence of a registrar who had to be paid. An Anglican vicar or curate was marriage celebrant and registrar in one man. The Catholic authorities in England recognised marriages in C. of E. performed before 1909 as valid. After 1909 a Catholic was required to marry in presence of a Catholic parish priest or his representative, after banns.
It's possible that some family members were baptised in a church of one denomination as infants and a different one later, in childhood, adolesence or adulthood. Adult baptism/reception might have been before or after marriage. Browsing a baptism register dated 1913-1917, I noticed several adults. All were married or about to be married. (People baptised after 1909 were supposed to have information about their marriages added.)