1) I'm not sure this will be possible. Correos require luggage to be dropped at hotel reception by 8am in order to be picked up and transported to your next overnight stop. I suspect Camino Facil will have similar arrangements.
Not sure what route the bus takes but if it happens to pass through and stop at Neda you could always jump off, drop your luggage at your overnight accommodation and then catch the next one to Ferrol or wander to the FEVE station at Xuvia and catch the next available train to Ferrol.
Last resort, pay for a taxi to transport the bag from Ferrol to Neda.
2) I printed at home and laminated the paper for waterproofing. It is worthwhile printing a few copies so if the label happens to be lost in transit you have a ready made replacement without having to find somewhere to print whilst on Camino. That said most accommodations have Correos labels that you can fill in by hand should you need to.
In the first instance, I would be inclined to email
paqmochila@correos.com explain your travel arrangements and dates and where you need the luggage to be and on what day and see whether they can accommodate it. Correos do offer a service to transfer luggage to any Post Office along the Camino route but whether this is a same day service I'm not really sure. You obviously want to avoid your Mum's bag arriving in Neda on the day that you have walked out of it.