It does _not_ end the round. So if the marines use an action card that empties a blip pile, after you resolve all of their actions, you move to the new location. So you'd place new terrain cards, and if the new location has a special effect that happens upon arriving, you'd resolve that. Then, you'd draw the event card, use it's effect and spawn and move genestealers as directed. Only then would you go back to the start of the round and choose actions again.
You forgot the Genestealer Attack phase.
So in this scenario:
- finish Resolve Actions Phase
- place new Location and Terrain cards
- discard remaining blip pile then create new blip piles
- resolve new location "upon entering" effect
- RESOLVE GENESTEALER ATTACK PHASE
- resolve Event Phase
Obviously if you've manged to wipe out all Genestealers on the formation with your Actions plus any Door Terrain tokens, then there will be no Genestealers that Attack during this phase. But if there are any hanging on - you still must have that Genestealer Attack Phase.