non-responsive surface
The following description is about this sculpture by Rachel Whiteread: https://www.nga.gov/collection/art-object-page.131285.html

1. Does "subtle" here mean "not harsh, not very direct, and not intense"?

2. Does "inverted fireplace alluding to the funereal" mean "inverted fireplace reminds of a coffin"?

3. Does "its non-responsive surface" mean "its flat and formless surface"?

Description:
Ghost, 1990, a plaster cast of a living-room, is more subtle in its preservation of the past (compared to House, 1993), but nonetheless the faint traces and stains of what was once inhabited by the living remind us eerily of our mortality, the inverted fireplace alluding to the funereal, its non-responsive surface seeming to
look back at us from a time past.

Question

non-responsive surface
The following description is about this sculpture by Rachel Whiteread: https://www.nga.gov/collection/art-object-page.131285.html

1. Does "subtle" here mean "not harsh, not very direct, and not intense"?

2. Does "inverted fireplace alluding to the funereal" mean "inverted fireplace reminds of a coffin"?

3. Does "its non-responsive surface" mean "its flat and formless surface"?

Description:
Ghost, 1990, a plaster cast of a living-room, is more subtle in its preservation of the past (compared to House, 1993), but nonetheless the faint traces and stains of what was once inhabited by the living remind us eerily of our mortality, the inverted fireplace alluding to the funereal, its non-responsive surface seeming to
look back at us from a time past.

Answer

Ok keep in mind that sculpture and art descriptions often take great poetic license with the meaning of the word they use. In other words they don’t intend to mean precisely, or literally,  what the words themselves mean. This one isn’t as bad as some.

1) yes basically. In this use it means not as bold or as intense as the other.

2) not exactly. Funereal is an unusual adjective that means having a tone or character appropriate for a funeral. So basically somber, mournful, etc. so the fireplace is meant to bring feelings and thoughts related to that.

3) I’m not as sure what the description is getting at there. It could mean literally when pressed the surface doesn’t have any give or softness.