It all depends on the context and, in your examples, on what follows. For instance, as regards Ms Melegatti: if that's the end of the conversation (implying that she is still addolcita), the passato prossimo is the only possible tense to be used. You'd use the trapassato prossimo if something like this followed:
– La maternità l'aveva addolcita, ma poi l'hanno licenziata e da allora è ridiventata scontrosa.
Even without a ma poi..., the phrasing with the trapassato seems to imply that she is no more so sweet.
As for the first example, see the difference between:
– Io ti avevo avvisato, ma tu hai fatto lo stesso di testa tua.
and
–Io ti ho avvisato: adesso fa' come credi.
So, you are perfectly right in assuming that trapassato prossimo denotes “un evento compiuto prima di un altro evento del passato”. As for passato prossimo, that it “sounds ... as something that happened now or recently, like a minute ago” is a common misconception, even among many Italians (the other being that passato prossimo is more or less the only available past tense in Italian). It is instead used for any past event that has still effects on the present. You might say nel 2008 Carla è partita per Londra, meaning that she is still there. If your Italian is good enough, you can have a look the article about it in the Enciclopedia dell'Italiano and related articles on other tenses.
