I haven't even done any "past tense" lessons yet... isn't the -amos ending for both present and past tenses?
Yes, it could be "we speak" too.
The same rule applies in Spanish as well.
-amos is to indicate plural. The past tense would be falávamos (so "-ávamos").
"Falamos" can be either present or past tense.
When written in the simple past, shouldn't it include an accent mark as in "falámos"?
This is the case for PT-PT. For PT-BR, the it is the same.
"-ávamos" is a past continuous, while "-amos" is eighter present or simple past.