The sort of routes which might provide an answer are not the way to go if you are using a paid researcher as they are time-comsuming.
- Look at all the wills for the village, in case Mark Marshall is mentioned in other wills
- ditto for land documents, to see if he signs as witness anywhere
- ditto manorial records, to see if he signed those, appeared on rent rolls, transfered land etc
- ditto land tax assessments, to look for the name changes
As OC says, sometimes we simply cannot go any further. The great Michael Gandy (appeared on WDYTYA, writes books, lectures etc) has two burials in a village, father & son, but no means of telling whether Dad died of grief after son's death, or if father died first. And it's no comfort to know that the vicar knew which one was which!