Depends what you define as few days.
If you leave it for 2-3 weeks it'll probably go flat so I'd get a trickle charger.
A week to 10 days will be fine.
Just tell them youre happy to cancel the sale and they'll register it.
There's no reason it shouldn't be registered first and then you can collect it after the insurance is sorted
The screen dimming is normal to prevent glare. There is a setting to stop this from happening or increase the screen brightness when it does this
On the homepage click on the Display button
If it's anything like the new RX the washer fluid comes out of the wiper arms rather than the bonnet and sprays near the wipers rather than the windscreen