All I can say is "It's about time"! The inflexibility of the existing rule must put many off buying.
In order of preference, 3, 1, 2
Option 3 would benefit anyone with work or other commitments that mean they cannot be home. Opting for delivery to a parcel shop at the point of order puts the member in control, allowing them to collect when convenient, knowing that their selected parcel shop may open early morning and/or late into the evening. This would also give anyone that lives at a "non-standard" address the ability to purchase a phone.
Option 1 could include an automatic redelivery to the selected parcel shop next day after one failed delivery attempt, saving the member having to get home, find the failed delivery card and then log on to rearrange the delivery, which could delay the delivery by a further 24 hours.
I don't like option 2 at all, as I can see it being fraught with problems, especially on those occasions when the dispatch confirmation is delayed and doesn't arrive until the day of delivery, making it too late for "in flight" changes.
As an aside, I've never quite understood the logic of insisting the delivery can only be made to the cardholder's billing address for security, and then giving UKMail licence to deliver to a neighbour (or anywhere else the driver fancies, it seems).
As for your final question "is it something you'd trust with such a valuable item as a phone?" surely that's a question giffgaff need to be asking themselves. The Consumer Rights Act 2015 makes the retailer responsible for goods until they are in the physical possession of the buyer, or in the possession of someone appointed by the buyer to accept them. This means that retailers are liable for the service provided by the couriers they employ (which I believe would extend to the parcel shops holding the goods pending collection)