I was very grateful for the assistance I received on this post. I thought people might like to read where it took me.
I am quite satisfied that the John Mercer named in Thomas Dadd's will is the same as John Meecey who married Sarah Bing at Deal in 1783. The will provided good clues: Sarah's mother Elizabeth was Elizabeth Dyason, probably the sister of Sarah Dyason who married Thomas Dadd.
One of the witness to John and Sarah Meecey's marriage was Mary Allen, who later married Sarah's brother Joseph Bing named their first child Joseph Dison Meecy and their second Elizabeth Dyason Meecey; a later child was named Benjamin Mercey, presumably after the brother who was cut out of the will. In the meantime they have several other children baptised under the name of Masey
The evolution of the Macey name is quite interesting as it follows a pattern with the changes in presiding clerics or parish clerks. Joseph Dison Meecey marries in Deal under that name, but most of his children are baptised in Walmer under the name of Mercer. (Not all: but the clerics in Walmer seem to change quite frequently). This would have been around the time that Thomas Dadd was making his will, so I am making an assumption that the clerks doing the writing for him would have picked up on that form. By the time of the 1841 census the pattern is more fixed: family members living in Deal are known as Macey, and those in Walmer as Mercer.
As a by the way, I think the James Dadd referred to in one of the posts above is actually Thomas Dadd's brother, rather than his father. This makes the beneficiaries cited in Thomas' will as his 'cousins' his nephews, but I think this may just be an older usage of the term cousin.
I am, however, still struggling to pin down a precise birth for John Meecey. I am gradually eliminating people from the surrounding parishes with variants of the name. It is a long haul, and I still keep coming back to the Maceys of Wye.
Thanks everyone for your help.
Betty Macey