1) you can choose both of them, however, the meaning changes. "Todavía no han llegado porque no se oye nada". - You deduce they have not yet come because it's no sound. "Todavía no han llegado, por eso no se oye nada". You supposse (or you are sure) they have not yet come, as a consequence it's no sound. It depends on where you need to put emphasis. 2) Yes 3) both: sin embargo: you expres contrast. In spite of the fact the car has some positive characteristics (according to the speaker) it's cheap (which it's not very usual). además: in addition. In this case you don't expres contrast. You are saying different positive features. 4) Yes, you can do it and native speaker do. However, when you say "sin embargo" the opposition or contrast is partial. If you use "en cambio" the opposition is total (but not all the times). "yo soy español, en cambio tú eres chino". It's a very difficult question even for native speaker. Currently people use both of them as synonymes. You can use "pero" as well.