Soft pack luggage on carbon seat post and handlebars

I'd have no worries about apidura seat pack on carbon post, most of the weight will be taken by the saddle rails anyway, so as far as the post is concerned it'll be just like someone 5kg heavier is riding.

On the aero bars issue as long as the straps are ling enough should be ok.

I've used an Alpkit seat pack on my Defy Carbon, and an Arkel tailrider on a radonneur rack (which attaches to the saddle rails).  Both were fine.

Agreed, you will be fine. As above, one thing i did was apply a single layer of gaffer/gorilla tape to the post and top of the frame (where seat post enters) before attaching back to stop the carbon and paint getting waron off / scratched

I'm sure you'll be fine - they take your weight riding, so a couple of extra kg should be within tolerance. Just make sure you tape up the contact points to stop any damage to the carbon.

Only time I've loaded up a carbon bike with luggage was a dry bag on a rear rack.

No rack mounts on that frame, so used a rack that mounts using the QR skewer and zip-tied the other bits to the wishbone above the rear brake with a bit of old innertube for padding. I've since found you can get a seatpost clamp with rack mounts, which is a better solution.