I am the UK citizen and will apply for my US spouse via savings route. 3 questions.
1/ After we've applied, is she allowed to visit the UK during processing or will they hold onto her passport until approved?
2/ Is the time to move to the UK 90 days or has it reverted to 30 days?
2/ We don't have family to stay with and will need to rent an AirBnb temporarily until we find a flat through an estate agent. What's the minimum length of AirBnb rental we can put on our application to satisfy the accommodation requirement?
Many thanks.

She will be allowed to visit.
Time to move is still currently 90 days
I,personally, would recommend 2 or 3 months. A couple of weeks would probably not be sufficient.

1) They’ll retain her passport. You can often pay to keep it but it’s not advisable to visit whilst waiting for a decision as if your application is granted whilst you’re in the UK then it’ll be technically null and void.

2) The entry clearance vignette is typically valid for 90 days.

3) Ideally six months. Remember you don’t have to stay at that particular property after the visa has been granted so consider choosing a property with the flexible cancellation policy.

Regarding #1, what would be the reason for the application potentially being null and void; are you not allowed to visit during this time? Thanks

Because you are applying for Leave to Enter as a Spouse which cannot be granted if you are already in the UK with Leave to Enter as a Visitor. There’s nothing stopping you from visiting assuming you retain a passport but it’s not a risk I would take. Pay the extra for Priority processing rather than a return flight.