No sorey, it wouldn't. You see "öppnades" is the past passive tense of the verb öppna. So the translation of "Alla breven öppnades" would be "All the letters are opened"

Good job anyway, got me thinking as well and swedish is the second official language in my country :D

Both are common, as in e.g. alla barnen.

Yes, the simplest way to see this is that öppnade = opened and öppna = open.

Brev is an ett word. Ett brev, brevet, flera brev and breven are the forms for singular, singular definite, plural and plural definite.