hmmm... Dele/dela/deles/delas don't follow the rule "possessive-conjugated-the-same-as-number"...
So we have:
- His car = Seu carro / o carro dele;
- His cars = seus carros / os carros dele
- Her bag = sua bolsa / a bolsa dela;
- Her bags = suas bolsas / as bolsas dela
- Their bags = suas bolsas / as bolsas delas.
So, if you use dela for her, it will never come in plural, for their, it will always be deles/delas
I don't see how we can be expected to know that it is "delas" and not "dela" here without context. I mean does "dela são" sound any different from "delas são?" In other words, if "As bolsas dela são roxas" is a legitimate sentence structure, and both "delas são" and "dela são" sound alike, we cannot tell if it is "their" or "her" without context. So, 1) do the two sound different? Can you hear separation between the S at the end of delas and the S at the beginning of São? 2) Is As bolsas dela são roxas" a valid sentence structure?
At slow speed it's obvious, of course, but the goal is to learn to differentiate at normal speed. There's so much elision between "delas" and "são", though, that there's no good way to differentiate without context. Both should be allowed in a situation like this, because otherwise it's just a question of rote memorization.
Yes, "As bolsas dela são roxas," is a valid sentence. It means 'Her purses/bags are purple'.
'As bolsas delas são roxas', means THEIR purses/bags are purple.
Maybe try reading this exercise aloud; your ear will become used to the sound of your own voice running the letters together--in this case the 's' from "...delas são...", so that it may sound (to some) like one word.
'Delas' just means two individuals of female gender. Although it would in most cases refer to women (or girls), because Portuguese is a gendered language, it's also possible that it could refer to two animals or inanimate objects with female grammatical gender ('bolsa' can just refer to a generic bag, or a small, soft-sided pet carrier). The same would apply to masculine people, animals, or items in the case of 'delos'.