Господин и госпожа are in use, although these are very official and wont be used anywhere but tv, goverment or whatever. most likely you won't hear them at street, in a bar or in a bus. "потому что" и "так как" are similar, they both refer to the cause of some event/phenomena. but the latter sounds better than the first one in the beginning of your sentences. e.g.: [так как]/[поскольку] бутылка была пуста, я лёг спать. ("потому что" sounds artificial in this position).

yes, these words are still used. 'Gospodin' and 'gospozha' were kind of forbiden words in USSR , but its dissolution, these two came back and now are commonly used instead of 'tovarisch'(=comrade) Well actually it is more of a written and diplomatic/business style. You probably won't face them a lot in the spoken language. Though you can here "Дамы и господа", it's "Ladies and Gentlemen" in English. Вы можете использовать и "потому что", и "так как", зависит от контекста.

"потому что" and "так как" are synonyms and are interchangeable in almost all cases. "потому что" never goes in the beginning of the sentence whereas "так как" may. some times you can see "т.к." in texts which is a shorter form of "так как".

Господин и госпожа - obsolete words

By the way, correct is "когда я могу использовать..." After verbs "могу" or "хочу" must go infinitive. As well as in "quiero usar" or "want to use" -> "хочу использовать" :)