"Sorry, but there are no plants here."
Translation:Sori, ond does dim planhigion yma.
Yes, and it is accepted as an answer, as is Mae'n flin gen i (and gyda fi instead of gen i).
In this context, not really. Sometimes fan hyn might be used more emphatically to mean 'just here, right here, etc', but a tone of voice might be more effective for that.