I can't find another David of the right age, and I can't find a Thomas of the parental age. I also can't find many baptisms, marriages, burials; so not doing very well with this one!
In 1851 this John and Mary were in the Halstead Union Workhouse and further down the page was Joseph born 1826 Stamborne, perhaps another son, but can't find any baptisms. Their presence in the Workhouse would have been enough to split the family: indeed, Abraham was a lodger and there were several other singletons in Halstead. I guess you may be lucky later on in that a widowed parent might be with a son or daughter.
As to the handwriting, as you say the usual abbreviation for John was Jno, but as with any enumerator, you can't rely on them sticking to the custom.
There's a possible marriage for Maria in Jun 1849 Halstead vol 12 p163, looks like George Dorking was the groom, they are living with George's parents in Toppesfield in 1851 - does the marriage pop up in the Parish Registers? Failing that, looks like down to the purchase of the Marriage Certificate to find out :(