Wasn't vs. Weren't

Solution 1:

As far as I'm aware you can never use "was" with "we" in proper English.

You're right: the verb be never takes the form was when the first-person plural pronoun we is the subject. Was is used only with the first-person pronoun I and with third-person singular subjects. (The preceding two sentences are true "in proper English", as you put it: the use of was with other kinds of subjects in dialects other than standard English isn't relevant to your question.) The answer to your specific question is that simple.

The complicated questions about when to use was vs. were deal with clauses where the subject is I or a third-person singular pronoun or noun phrase.