Answers to some of your questions below.
1. How does the terrain differ between the final stage of the Frances and the Ingles - more ascents/descents? Is there much walking on tarmac for example?
I've not walked the Frances but I can say that the Ingles is undulating with a few steep pitches but the steep hill that many used to comment about between Betanzos and Bruma has now been by-passed and is a lesser incline and cuts the stage by several kilometres. There is a considerable amount of tarmac but there are some really pleasant off road stretches. In all it is a really nice route, particularly the first 3 standard stages.
2. How easy would it be to walk the Ingles in stages of not more than 25 kms per day over a week?
The route is not abundant with accommodation so you may need to walk a little more or a little less than 25km to fit or use taxi to and from. I think that there was another thread on this so will try and find it and post a link.
3. How easy is it to do a self-guided walk - is the camino well signposted?
Very easy, the route is very well waymarked.
4. Accommodation. Last time our accommodation was booked for us in a mixture of 2 and 3 star hotels, close to the camino. Part of the reason for this was that the group was half visually impaired walkers, half sighted guides, so accommodation needed to be fully accessible, so we used an agency to plan our camino. How easy would it be next time to book suitable accommodation direct for a group of around 12 people in twin bedded rooms? I am a light sleeper and don't do hostels/dorms.
You can stay in private accommodation throughout the route. I would recommend booking as soon as possible if you have a large group as the Ingles seems to get busier each year.
5. Can we arrange to have our luggage sent ahead for us to the next stop? How would we book this.
The Spanish Post Office run a luggage transfer service. Currently €20 from Ferrol to Santiago.
https://www.elcaminoconcorreos.com/es/transporte-mochilas
6. Transfers from airport to start of walk - any advice?
I took a group of 10 on the Ingles in 2016 and I actually booked a private transfer with Monbus. They collected outside the terminal in Santiago and dropped right by the Hotel in Ferrol. Although we were only 10 they gave us a full size coach so plenty of room. €22 each from memory. It was only 5 or 6 more than the public bus I seem to remember once you have caught the airport bus into Santiago and then the Ferrol bus.
http://www.monbus.es/es/alquiler-vehiculos-con-conductor