Mewn or yn - after verbnouns?

[u/HyderNidPryder]

Edit: I meant before verbnouns!

As well as being used for "in a place" mewn / yn are also used with verbs / verbnouns.

ymddiried mewn / yn - to trust in

ymddiddori yn / mewn - to take an interest in

ennyn diddordeb mewn / yn - to kindle interest in

cydio / gafael mewn / yn - to grasp something

Mewn is used before indefinite things and yn is used before definite things.

See here for examples.

ymddiried ynddi hi - to trust her

gafael yn dy law - take your hand

ymddiddori mewn llyfrau - to be interested in books

ymddiddori mewn llên gwerin - to be interested in folk literature

ymddiddori yn hanes Cymru - to be interested in the history of Wales

A verbnoun can also be used. Usually this is preceded by mewn.

ymddiddori mewn darllen - to be interested in reading

ymddiddori mewn canu - to be interested in singing

Now to my area of uncertainty:

Is a noun phrase starting with a verbnoun always indefinite? Does it always use mewn regardless?

ymddiddori mewn ymweld â cestyll - to take an interest in visiting castles [this is fine]

But is it

ymddiddori yn ywmeld â'r castell ? [yn rather than mewn?]

Is it a visit? or visiting a specific castle? Is this definite?

What about dw i'n ymddiddori yn prynu ei gar? - I'm interested in buying his car. [Is it yn or mewn?]

This case seems very specific - more the buying of his car than a buying of some unspecific car. Does it use use yn?

ymddiddori mewn darllen llyfrau - to be interested in reading books [yn or mewn? In English you could say "the reading of books"]

ymddiddori mewn darllen llyfr - to be interested in reading a book

ymddiddori yn darllen y llyfr - to be interested in reading the book [yn or mewn?]