Hi Rob,
That sounds like a fantastic plan and a very memorable way to experience the north coast. It's clear from your post that you've already done thorough research, and your proposed itinerary is one that many experienced walkers successfully follow.
A few thoughts based on experience and common feedback:
1. 3 days/2nights is very doable for experienced walkers, though it may feel full. If you enjoy stopping and taking photos, cafés, and longer breaks, adding a fourth day would give a more relaxed pace to allow for that - but your plan isn't unrealistic.
2. Direction: walking Portstewart Strand to Ballycastle is generally preferred. You finish with some of the most dramatic scenery (Carrick-a-Rede, Ballintoy, White Park Bay) rather than tackling it early when you're feeling freshest.
3. Daily breakdown: what you've suggested is very common and a sensible split. I'd recommend taking a look at this page:
https://bit.ly/2WOiRTU, where you can tailor the filters directly to what your accommodation preferences are.
4. Bag transfer: Options are quite limited. Some walkers arrange local taxis or accommodation-to-accommodation transfers, but there's no large-scale service. Planning to carry your backpacks or organising a private transfer in advance is safest.
5. Many walkers choose to use a mix of public transport. Here is a page dedicated to public transport that may be helpful to you:
https://goo.gl/wUMEuN. Trains and buses from Belfast to Coleraine are straightforward, but Ballycastle connections are a little less frequent, so I suggest checking timetables carefully.
6. General advice: As you mentioned, the weather can be variable, so make sure to pack appropriately. It can get quite windy at times. Some cliff sections are exposed, so good footwear and caution in poor visibility are essential. Cafés and shops are quite spread out between villages, so make sure to bring enough fluids and snacks.
It sounds like you're already very well prepared, and late August is a great time to do it. I hope you enjoy the walk - it's really one of the most beautiful coastal routes on the island.
Let me know if I can offer you any more advice or help in any other way.
Best,
Sadhbh