I have come across an interesting article that could possibly explain the Trewren alias Michell situation.
"Some Devon Surname Aliases" by Mike Brown.
genuki.cs.ncl.ac.uk/DEV/DevonMisc/Aliases.htmlBrown argues that there is little evidence to support the theories that aliases were used to denote illegitemacy or to indicate a family's place of residence. However, he argues that there was a convention that aliases were used to preserve the previous names of females who had been named in property leases or wills and who had subsequently married. The alias form could also be applied to the children of the marriage to clarify their inheritance position in relation to their parents' families. This would explain why the alias form often disappeared after a few generations, when it no longer had any relevance.
My theory on the Sancreed "Trewren alias Mychell" descent is set out below. This is, of course, pure speculation and is bound to have some errors, but I believe it is a plausible explanation. (I have only included the lines of descent with the "alias").
(1) Thomas Trewren alias Myhell
11 Feb 1570, Mar, Thomas Trewren alias Myhell & Jenat
6 Apl. 1609, Bur, Thomas Trewren alias Michell
(2) 27 Aug 1573, Bap, Thomas s. of Thomas Mychell
29 Jan 1603, Mar, Thomas Michell alias Treuren & Jane
(3) 12 Dec 1604, Bap, Henrye s. of Thomas Michill jnr.
(3) 23 Mar 1607, Bap, Walter s. of Thomas Trewren the younger alias Michell
(3) 15 Nov 1609, Bap, Richard s. of Thomas Trewren
(4) 10 Oct 1637, Bur, Marie d. of Ric. Trewen alias Michell
(4) 27 Apr 1639, Bap, Thomas s. of Richard Trewren alias Michell & Elyzabeth
(3) 31 Dec1612, Bap, Tamzen d. of Thomas Trewren alias Michell
(3) 1 Aug 1615, Bap, Elyzabeth d. of Thomas Trewren alias Michell
(2) 6 Apr 1576, Bap, Willyam s. of Thomas Trewren
(3)16 Mar 1601, Bur, Son of William Michell alias Trewren not being baptised
(3) 23 Feb 1619, Bap, William s. of William Trewren alias Michell & Elizabeth
25 Jun 1581, Mar, Thomas Trewren alias Myhell & Jane
(2) 6 Oct. 1584, Bap, Francis s. of Thomas Mychell
5 Mar 1666, Burial, Francis Trewren alias Michell
(3) 30 Jan 1606, Bap, Christian, base s. of Frannces Michell alias Trewren
(3) 1 Aug 1610, Bap, Argent d. of Frannces Michell
17 Apr 1637, Mar, Argent Trewren alias Michell and Richard Trembah
(3) 24 Oct 1613, Bap, Gabriel s. of Frannces Trewren alias Michell
(2) 26th Dec 1594, Bap, Stephen, s. of Thomas Treuren
04 May 1619. Mar, Steven Trewren and Jane
(3) 27 Feb 1622, Bap, Thomas s. of Steven Trewren alias Michell & Jone
The only other "Trewren alias Michell" record I can find is the burial, on 13 June 1674, of Alice Trewren alias Michell.