Can you clarify these plot 'issues'?

Hi, Some things bother me with Alien/ Aliens... Can you clarify if these are genuine plot issues? - In Aliens LV-426 is to be colonised. Yet this moon (or rather the spacecraft on the moon) was giving off a warning signal that the Nostromo picked up on easily. Why would the signal not have been picked up by the terraforming team/ colonists? If so the derelict would have been discovered much earlier. - In Aliens, Burke says the suspected alien on LV-426 is a 'Xenomorph'. Doesn't this imply that this type of alien has been discovered before, and not just by Ripley and the Nostromo crew? This could be a nice tie-in to Prometheus. How does Burke know about this species? Another thing. Many people are hypothesizing that the SJ ship in the trailer is the one from Alien. But this ship has been on LV-426 for a LONG TIME. So how can it be the one from Alien? Why does LV-426 need to be in Prometheus at all? Comments appreciated. Melk