I don't know if you can call this answer "masterful," but here goes.
This article (entitled: "Snatch," "Hole," or "Honey-pot"? Semantic Categories and the Problem of Nonspecificity in Female Genital Slang.) is quite an extensive study on many, many statistical phenomena and anomalies when it comes to, as they call it, female genital slang. They also compare these slang Female Genitalia Terms (FGTs) by category to Male Genitalia Terms (MGTs).
This article is very extensive, so to highlight what they have to say about the term "tuppence" (which here falls under the "money" category):
FGTs contained both explicit (e.g., tuppence, thruppeny bit, Mrs Penny), and implicit (fur purse, pocket book) references to money. In most terms, the amount of money was very small, suggesting reference to money rather than to value. Many FGTs not coded with this category (e.g., fish, lettuce, quiff) have, historically, meant money (Wentworth &. Flexner, 1975), and many have simultaneously meant prostitute--Green's (1999) money category is identified as the money-maker. These terms suggest women's worth and value to be in their genitalia, and commodify the genitalia as objects to be purchased. Indeed, commodity was a sixteenth century British term, now obsolete, for the genital area (McConville & Shearlaw, 1984).
Thus, as @Garet Claborn intimated, this term seems to derive from referring to prostitutes, specifically cheap ones, and as they say points to women's worth (at least the opinion of the times) being in their genitalia.
With reference to your mention of whether or not this word enjoys usage, as you say a quick Google search will yield a number of hits connecting the word "tuppence" to a female genitalia reference. As for how widespread it is, a discussion on this forum suggests that it's not a very widespread and widely known word, one user saying the following:
I'm guessing that as Mummy, Walt Disney, Agatha Christie and my other half (parents from West London, raised in various locations across Europe) and the Online Oxford English Dictionary do not know the "front bottom" meaning [referring to tuppence], its geographical spread is limited.
Hope this answers your question.
It varies. Both of your examples might indicate a sexual or romantic interest, or it might not, depending on the proclivities and status of the speaker; alternatively they might be general expressions of excellent in the object. It might or might not refer to excellence of physical appearance, but I think that is not usually the primary meaning.
With inanimate objects (including plants and the like), it most likely refers to excellence in the sensual or aesthetic qualities of the item.
Best Answer
The most common uses are probably to describe people (and to a slightly lesser extent, objects), with the meaning "very ugly" or "repulsive". Less common but still frequently it can mean "smelling very bad".
It is also found frequently as a verb ("That food mings"; "She mings to high heaven"), with either of the senses above, and also commonly with an "-er" suffix to denote a person not possessed of physical beauty ("That psmears is a right minger").