As with most other literary techniques, I think a lot depends upon the skill with which a POV shift is done. But generally a shift in mid-paragraph in an otherwise straightforward piece of prose will be read (by me, anyway) as clumsiness rather than intent.

Yes, I completely agree. Clumsiness or even unawareness, if a new writer, or lazy if someone's been writing a while.

I don't think I've written anything in third person omniscience. Maybe I should try! Bit daunting, though; I suppose I'd rather be in the head of just the one character.