I think Phoebe saw or heard something she wasn't meant too and needed to be silenced or got in middle of a DV that got out of control. Doubt they would kill her over unpaid rent or bills.
Same with arguments, if they wernt getting along & annoying one another her leaving resolves this.
The last (unlikely) a sexual relationship with housemate/s that went bad or a encounter ended badly.
1st one makes most sense since she was leaving the state and no longer under there control.
Makes no sense or any good reason why she was dropped off near and not inside the airport.
Unless someone "credible" has confirmed hearing or seeing Pheobe on the 15th its very likely something happen to her on the 14th and what the housemates say happened on 15th is BS.
Love to see cell tower info for Pheobe's, TB & JW phones and the housemates call logs.
PB was there for a month.
According to the neighbour there were noisy parties there, dogs howling, etc. during the time PB stayed there.
So the girl obviously allegedly knew what was happening there
and didn't report them.
Allegedly
never notified authorities.
Nothing happened to her during this month.
Only before the flight.
I doubt she was suddenly blackmailing them about reporting the pair about something unlawful.
But there seemed to be alleged noisy argument just before her leaving.
That is why I suspect it was about settling matters among them, maybe financial.
Tempers might have flared.
She was leaving,
but some issues might have not been resolved.
Re "got in the middle of DV"
(quoting OP)
TB wasn't living in the house any more.
But in a bus.
It might also allegedly have happened
that
her disappearance was pre-planned for this particular day of the trip.
But then,
why to argue noisily?
Why to draw neighbour's attention to themselves?
Who knows?
All just speculation on my part.
JMO