Regarding the setting of the phasers in the final fight at SF HQ, we should note that it's something neither Crusher nor the one person told by Crusher to use "kill", Picard, would be in a position to choose.
The phasers used in the fight were taken from the possessed people. The one possessed person who retained use of his weapon, Admiral Aaron, fired at our heroes but missed; the beam created a bright flash but did no damage whatsoever
to the wall it hit, or to the painting on that wall. It would appear the phaser was at most on one of the "heavy stun" settings, because "kill but don't vaporize" in the 24th century seems to leave scorch marks.
Riker in turn beamed down some time after Crusher had his discussion with Picard. Quite possibly, new information was uncovered, and Riker knew there was no need to use the kill setting to subdue the possessed people; heavy stun would do.
When the time comes to destroy the Remmick-shaped creature, it's Riker's gun that does physical damage; Picard's bigger sidearm, confiscated from the conference room , merely creates the sort of chest glow typically associated with stun. So we could well assume that Picard isn't touching the settings, while Riker has enough extra information to know when to key his personal phaser up or down.
Why Quinn goes on a rampage could have two basic explanations:
a) It's Quinn fighting, not the parasite. Quinn is ruining the plan by trading blows with Riker; the parasite is for some reason powerless to stop this, perhaps because Quinn has confused it into thinking that the fisticuffs would be a good idea, perhaps because Quinn has somehow gained the upper hand.
b) The idea is to both eliminate Riker and other top officers, and to lure in Crusher to treat them. Quinn's body is expendable; by beating up Riker, jamming his ability to call Security (an Admiral could do that easily enough, I trust), and instead sending a message that lures in Worf, LaForge and Crusher, the parasite eliminates a number of threats while taking over a "lucky survivor" who can subsequently ease other parasites aboard.
We do know that the possessed people aren't very convincing in their imitations (who would believe Remmick's words about peaceful coexistence when delivered like that?), while OTOH the parasites should know a lot about mankind and about what it would really take to manipulate our heroes. It would thus be possible to argue that the parasites have poor control of the host bodies, and it is for this reason that they generally fail to convince, and specifically fail to control Quinn during the fight.
On the other hand, if Quinn did triumph over the parasite, this means the critter apparently only managed to influence Quinn's face and speech, but couldn't control his limbs. Odd for it to go that way... And why would the parasite bother to invent rationales for Quinn's undesirable behavior ("I enjoy
beating you up!")?
Assuming that everything did go according to plan is fairly easy. The Quinn creature had eliminated Riker and LaForge as threats, and was going to accomplish that with Worf as well in pretty short order. Then it would be time for Crusher to arrive, be subdued, and for the parasite to be injected (we don't know how that happens, but supposedly it's fairly complicated and best accomplished by luring the victim into a locked room and subduing her conventionally - fist in the jaw, phaser on stun).
The parasite side of things makes sense, then. What does not
make sense is that Worf and LaForge arrive "unawares", walking right into the trap - but Crusher comes in armed, something she basically never does otherwise. Being unprepared for such a startling twist of events does not make the parasites look stupid. It does make one wonder what sort of a message Quinn sent out that was going to lure the victims in unalert but running
, and the doctor in unarmed and unawares...
Regarding the "Angel One" issue, an unknown disease is not that big a problem if the idea is to beam up the Odin
folks. It only becomes a problem if there is going to be any beaming down going on; in that case, an entire planet might die.
However, it doesn't seem as if Crusher would have a reason to think that the deaths on the planet are particularly imminent. Indeed, hurry would not be a factor: Mistress Beata is only going to execute the Odin
crew if they refuse to leave. Them being unable
to leave for the next few days because of a force majeure would not be a problem for Beata.
Crusher doesn't condemn anybody to death. And Beata only does so after Ramsay declares he wants
to be executed rather than deported.