Let's start with the possible marriage of Elizabeth ............... and you have me confused again!!
You mention that Reginald Thomas was 'from Madron' yet Phillimore states that he was 'of Camborne'!
Taking a look at IGI for possible baptisms for Reginald Thomas and I see there are only two.
15th July 1753 Madron son of William
16th January 1755 East Newlyn son of Reginald
I take it then that you have identified the Madron baptism to be that of the man who married Elizabeth Glasson?
If that is the case then I might suggest you think more about the man from East Newlyn.
Reginald Thomas married Susanna Hawes of Lelant 30th May 1789 at St Just
I would think it might be more likely that this was the Reginald from Madron.
Next on the agenda must be Susanna Mitchel.
From the list available in IGI (after wading through the unnecessary rubbish!) it would seem there are not many possibilities.
She would be born no earlier than about 1709 and no later than about 1730 which gives 20 possibilities.
Cutting out the aforementioned 'rubbish' cuts that number in half and leaves 10 baptisms within the limits.
Of those we can certainly eliminate the two at Tremaine and we are down to 8.
The remainder are:-
1st January 1710 St Gluvias d/o Zach and Jane
7th September 1711 Gwennap d/o Arthur
12th February 1714 St Agnes d/o Philip
31st December 1715 Gwennap d/o Arthur
29th May 1718 Probus d/o Isaac
27th December 1721 Helston d/o Richard
19th February 1726 Landewednack d/o Ezekiel and Jane
22nd November 1727 Falmouth d/o William
Robert and Susannah only used three names that are readily identifiable for their children - Dorothy, John and Robert.
No daughter named Susannah which may mean that none of the other names come from the Mitchel side of the family.
They did name a son William and another Richard so the 1721 Helston and 1727 Falmouth baptisms might be considered.
But given the marriage occurred at Gwennap I would be leaning towards the 1715 baptism for the daughter of Arthur.
Best I can do for the moment.
CT