As this forum question states, which sites Palmer's The English Verb,
This depends on whether the statement being reported is still true for
him. If it is, he may (but is not required to) retain the present tense.
Taken out of context, it is hard to be sure but it is (probably) still true, for the speaker, that we need to stop littering in order to protect the planet, which implies that the present tense is appropriate. The quote also suggests, and this seems intuitively right to me, that "correct" tense usage in reported speech does not follow very strict rules.
Maybe this indicates that exam questions on reported speech ought to provide more context?
Finally, in your example (1) the tense and structure of the first clause is also interesting, which is reporting a lack of ability. "Could" is used for both permission and ability so in a written text this would be clearer:
She told us we wouldn't be able to protect our planet if we DON'T stop