Hello Momamg,
Have you tried to find a Will for either Elizabeth Best, or Charles Best? It might be worthwhile searching to see if you can find one for Elizabeth at least. That would give you precise details of her death.
There are many Family Trees on Ancestry for Charles Best and Elizabeth (Lillywhite). One of these has her death as 16 March 1825 with an alternate date of 16 April 1835. Both show place of death as Beeding, Sussex. There are others who show her death as 16 May 1825 or 16 April 1835, sometimes vice versa. In all, confusion seems to reign about her date of death. There is one tree which has her as dying in 1835 aged 74 which puts her year of birth as 1761. Some have her place of death as Sussex, nothing more but others show it as Beeding, or Upper Beeding.
Another tree shows Charles Best as being buried in Upper Beeding but your information indicates this is incorrect.
It is likely that Elizabeth would have been buried with her husband. She may have her name on a gravestone with his and the monumental inscription would provide details of her death. Have you tried to find a grave? The SFHG may have transcriptions of the gravestones there.
Since the details her son gave for the death of his father have proved to be correct, I suspect that those for Elizabeth written in the family Bible would also be right. It seems there is much confusion over Elizabeth's date of death and for all researching this family. This makes me think that you will need to establish where she was buried and whether there are details on her gravestone. The other possible source would be a Will.
What other parishes have you searched for her death? The Sussex Parish Registers have not all been transcribed and are not all online. Have you considered other neighbouring parishes?
The burial in 1835 in Hamsey doesn't look feasible to me. The age is wrong, plus a 10 year difference in her date of death. I cannot see how the registers could record a death that occurred in 1825, 10 years out of sequence. I take it that you have not looked at the prime source. The only way to check would be to examine the actual parish register.
Jo.