It can be all of them, and also physically (as in leaning against something). You can usually tell from the context.
Can all adjectives be used this way, as nouns? For example, could I say "Lubię czerwony" to say that I like the red color?
I am not sure about ALL adjectives, but you can use color adjectives this way for sure.
Depends on how you treat the word, but not here.
If you treat it as only describing poor women (or some other noun that is not masculine-personal), than the Nominative is "biedne" and Accusative is also "biedne".
If you treat it as describing either poor men, or just poor people in general (as here), then "poor people" include the possiblility that there are some males among them, thus the form has to be "biedni" (Nominative) and "biednych" (Accusative).