Untangling Possession in Cornish

My language project this year is Cornish (Kernowek, Kernewek),

I’m using Desky Kernowek by Nicholas Williams as my primary text. Having reached all of lesson two, I’m perplexed by possessives.

Desky Kernowek asserts that both of the following are correct usage for ‘the boy’s dog’.

ki an maw OR an ki a’n maw

Williams cites a phrase from Jowan Chy an Hordth
an ôst an tshei
and suggests that this would render as an ost a’n chy in modern spelling and is an example of the second construction.

This article has a different take, arguing that
1. an ôst an tshei is an example of English usage being absorbed into late Cornish
2. phrases like ki a’n maw indicate that the possessed item is in definite (a boy’s dog [ Ed. A dog belonging to the boy] rather than the boy’s dog)

Since Cornish was effectively extinct, the usual arbiter of grammaticalness is texts from the period when Cornish was a spoken language. Williams includes textual examples in each lesson, but his lesson two examples don’t include the an____a’n______ construction. “Avoiding Overdetermination” doesn’t appear to include source text citations,

My two questions given all that are…..

1. Is there any source text support for “Avoiding Overdetermination”‘s claim that _____a’n_______ indicates an indefinite possessed noun?

2. If you agree with Williams , are an ________ a’n__________ and _________an_________ interchangeable, or are there instances where one is preferable to the other?