I have little access to copies of Manx documents of this age at the moment (and my textbooks on handwriting are in Peel whereas I'm not) but I do have the 1819 copy of facsimilies of key English + Scotish historic documents which illustrate many hands - looking at the 1488 acta parl Jac iv dated 1488 the characters are definitely the 'thorn' letter as is the 1503 act, by 1525 th and thorn seem to be equally used - the same holds true of the 1585 act of Scots king James

I agree that the 16 and early 17th C 'h' looks more similar to our current g - maybe this has led some to transcribe y rather than the correct th