1 You can use it with other conditionals; Google gives many examples here: "unless i were - Google Search
It is less common, but its not an absolute rule.
2 In formal language the subjunctive ('were' for all forms) is preferred. Traditionally, it was regarded as the correct form. However, it is very common to use 'was'. Exams like Cambridge accept both forms. It is something that some people feel strongly about, so being aware that in formal contexts, the subjunctive is favoured is useful.