When you say "invulnerable NPCs," are you talking about the normal town NPCs or invasion NPCs (i.e., members of the invading army)? If you are talking about town NPCs, you do NOT need to kill them to keep the invasion going. Sonoma's had several towns invaded and a number of them had crimson dragons. I'm absolutely certain that killing the town NPCs had no effect on the strength or duration of the invasion. My understanding is that the invasion strength is tied into sigil control and/or defeat of the invasions themselves, and that the strength of the invading forces goes up the more times a town is invaded or the longer it remains invaded.
I checked earlier on Napa and it appeared that SL controls all the towns. I really think you need to see what happens if TB or COM can get control of one of these towns. I think you will see the townsfolk become uncorrupted and go back to their normal state. Then if SL or Minax takes the town again, you should get another invasion that should be stronger than the last one, possibly even including a crimson dragon.
If SL or Minax continue to have uninterrupted control of a town, then I think you will possibly continue to have low-level invasion forces spawn sporadically in that town. However, if they are quickly defeated and the town control never changes, that may be all you ever see unless the developers step in and force the invasion to the next level. I assume they'd rather see the players themselves do that through the faction side of things, with town control shifting between Chaos (SL/Minax) and Order (TB/COM).