In a BBC World Service Learning English answer about the before nouns, Roger Woodham comments:
It is normal to insert the before the adjective + noun in superlative sentences as there is only one of its kind in this category.
Generalising this idea, even if the adjective isn't a superlative, adding the before the adjective serves to give more prominence (power?) to the adjective, and hence the noun.
You use as an example:
He thought Amelia worthy even of the brilliant George Osborne. (Thackeray)
Consider a modification to use the zero article instead:
He thought Amelia worthy even of brilliant George Osborne.
The zero article version simply notes that George Osborne is brilliant. There's no sense that he outshines others who are also brilliant, or that brilliance is his defining quality (over perhaps tact or sportiness - I don't know the context), whereas the definite article elevates the adjective brilliant towards a superlative.
Regarding your numbered questions:
1 & 3. Is this a real rule? Is there any reason for this rule?
It's stated as a rule of thumb ("... most cases ..."). The reasons for rules of thumb tend to be empirical, rooted in the author's research and experience.
- Is this class of modifiers (old, young, poor, dear, little, honest, lazy) a closed set?
No. Insert the zero articled "lovely" before Amelia, and the sentence is still grammatical.
- Is it possible to say, at least in some contexts, "the young/old/dear/poor/little/honest/lazy Peter"?
Yes, use any of these modifiers in place of "brilliant" in your Thackeray example: the resulting sentence is still grammatical, and "the" still 'amplifies' the adjective. The definite article is also present in other contexts, e.g. "The young, the old, and the economists".