[Spellyans] Multiple adjectives after feminine nouns

[Michael Everson]

Caradar sets a rule that only the first of several adjectives  
following a feminine noun is

This is not what happens in Welsh ("y fasged bicnic goch fawr" 'the  
large red picnic basket') or Breton (ur gazeg vihan c'hlas 'a small  
grey mare') or Irish ("an bhean bheag bhocht" 'the poor little woman').

Caradar's rule:
If a word intervenes between noun and adjective, the latter remains  
unmutated: an venan yū cōth (not gōth), the woman is old; būgh wyn  
tēk, a fine white ow (gwyn is mutated, tēk is not).

Pool repeats this (Cornish for Beginners, p. 9):
"This mutation only occurs when the adj. immediately follows the noun,  
not when it is separated from it, e.g. by another adj. A good woman,  
benen² dha cōth (cōth not mutated).

Hal Wyn says similarle:
When more than one adjective follows a feminine noun only the first  
adjective need be put in the feminine (second state) -- benen dêk (ha)  
bras, a beautiful and big woman.

Brown and Williams follow suit.

The question is, where did Caradar get this rule? Is this behaviour  
(which differs from other Celtic languages) attested? it would seem to  
me that if it is attested, it should be easy to find in the corpus. If  
it can't be... shouldn't this rule be revised?

Michael Everson * http://www.evertype.com