Jose - some of this I will need to work on a little later but I can add a couple more details for you now.
From information passed to me years ago Edward Michell (husband of Martha Davy) was baptised at St Just 31st October 1857 son of Edward Michell and Mary (nee Rodda).
I think you had some of this but ....
The first known child of Edward and Mary was Mary who was baptised 1st January 1756 at St Just.
I have not followed up on this part of the family as yet so I don't know whether or not there were other children although a check now of Zennor, St Ives, Towednack, Lelant and Ludgvan produces no more.
What I did find earlier today was three more children for Edward and Martha. But I do have a query over one of them as I cannot yet find a burial.
Anyway - here are the children I have found for Edward and Martha .... including Edward.
Amy bp. 13th April 1788 Lelant
Elizabeth bp. 22nd May 1791 Towednack
EDWARD bp. 1st February 1795 Lelant
William bp. 10th September 1797 Lelant
William bp. 22nd January 1801 Lelant
John bp. 16th July 1809 Ludgvan (born 4th May 1809)
I am not sure about the marriage for Amy so will check on that a little later but Elizabeth certainly married William Dunstan and had 10 children.
Martha Davy was certainly a spinster when she married Edward Michell but I cannot be absolutely certain about Edward's status as there is no record either way in the Parish Register.
Martha was living with her daughter Elizabeth in 1841 and her age recorded as 70 so she was born probably sometime between 1766 and 1771.
Although they were at Lelant in 1841 I think this is Martha's burial:-
Martha Michell of Canons Town age 78 buried 1st May 1846 Ludgvan
That means she was born around about 1768 and would have been only about 19 when she married.
And here we go again with the variant names!
Martha daughter of Martin and Elizabeth DAVIS bp. 3rd July 1768 Lelant
It was probably her father Martin who was a witness to her marriage.
And her parents are undoubtedly:-
Martin DAVY and Elizabeth UREN who married at Lelant 9th April 1760.
I am not sure how you came to this conclusion
but Martha, as shown above, was only about 19.
And I now suspect that Edward may have been younger and also that he may not have been the Edward born at St Just in 1757!
He was almost certainly deceased prior to the 1841 Census and I am thinking that this is him:-
Edward Mitchell of Curlurryan age 71 buried 14th September 1837 Ludgvan
That puts his birth around 1766 .... if his age is anywhere near accurate!
Looks like I have found myself some more work to do!
Time for an episode of Rawhide so I will take another look at this problem afterwards.
I will also take a closer look at the Australian information you have supplied and see if that helps me track down John Michell.
Can't promise everything tonight but will try and do a little more anyway.
Yes please.
CT