Yes, I agree that the 1929 burial must be the correct one for the older Peter, father of Peter Ronald (sometimes recorded as Ronald).
MYLCHREEST, Peter Crebbin, 58, Douglas Borough Cemetery, 23 Mar 1929 [BL burials index].
MYLCHREEST, Matilda Alice, 43, Douglas Borough Cemetery, 14 Oct 1915. [BL burials index]
Peter's second name given at burial was his mother's name pre-marriage (Eleanor Crebbin), at the time he was baptised on 4 Dec 1870. This makes his age of 10 accurate in the 1881 census, as well as at burial.
It was only during his working life that he appears to have deducted 3 or 4 years from his age.
1871 census, from the scan:
Name/ Relation/ Condition/ Se x/ Age/ Occupation/ Where born
CREBBIN, Catharine Head Marr F 39 Fishermans wife Isle of Man Rushen
CREBBIN, Elinor Daughter F 18 General servt Isle of Man Rushen
CREBBIN, John Son M 11 scholar Isle of Man Rushen
CREBBIN, Catharine Daughter F 5 scholar Isle of Man Rushen
CREBBIN, Even Son M 1 Isle of Man Rushen
KELLY, John Boarder M 8 scholar Isle of Man Patrick
MALLACHRYST [as spelt], Petter Grandson M 4m. Isle of Man Rushen
Piece: 5778
Folio: 51
Page: 22
Registration District: Isle of Man
Civil Parish: Rushen
Address: Lengage Road? Rushen Isle of Man
Marion, in the IOM it was usual for a child to be given the father's surname, even when the parents didn't marry later as Peter's did.
Marriage FamilySearch:
NameJames Mylchreest
Birth Date1852
Birthplace
Age21
Spouse's NameEleanor Crebbin
Spouse's Birth Date1853
Spouse's Birthplace
Spouse's Age20
Event Date09 Oct 1873
Event Place Malew,Isle Of Man
I think Eleanor/Elinor Crebbin's parents were John Crebbin & Catharine Watterson (son Evan bapt Rushen 12 Dec.1869). You will find the family members between the IOM and Lancs in censuses.
NameJohn Crebbin
Event TypeMarriage
Event Date13 Jun 1850
Event PlaceRushen, Isle of Man
GenderMale
Marital StatusSingle
Father's NameWilliam Crebbin
Mother's Name
Spouse's NameCatharine Watterson
Spouse's Marital StatusSingle
Spouse's Father's NamePatrick Watterson
Spouse's Mother's Name
Digital Folder Number004500280
Sue