This may be your lucky day. I have just got back into family history after a break of some 12 years. Anyway last time I did research I planned to create an index of all Bunyans births, deaths and marriages. I completed the deaths and quite large chunks of births and marriages. Looking through these indexes this is what relates to your questions.
1. No Ellen Maria Bunyan born 1 1 1901 in Aylesbury but there is one Ellen Maria Bunyan born in the 4th Quarter 1901 in Wellingborough, which isnt a million miles from Watford. (my birth records are complete from 1897-1905 so I am pretty sure this is your nan)
2. Mary A Bunyan is listed as dying 3rd Quarter 1917 aged 76. This would give the 1841 you expect
3. The only death I have of a Maria Elizabeth is in Hertford 1870 quarter 1 aged 6 (my death records are complete from 1837 to 1988)
4. My birth records also cover 1871 to 1876 and then all of 1881 and there is only one Rachel Bunyan listed and that is in Thame, again the right part of the world. This may be your Rachel but you need to check a bit wider as she may be older or younger than my records cover
5. If Rachel never married then I have a record of a rachel dying in 1937 in Wellingborough aged 76 quarter 3 which would place her birth in 1861 ( infortunately my marriage records are too sparce for the appropriate period to rule out a marriage.