Hola
It is great to see so many people interested in the Camino Ingles - it is a lovely route. I understand your anxieties and even although it is a 5 day route from Ferrol it has some real challenges and a sense of achievement at the end.
Your questions:
1 Betanzos to Hospital de Bruma (Meson do Vento)
This is the longest stage with a 4 kms climb over to Bruma at approximately 18kms from Betanzos. The route is generally flat and manageable apart from the steep climb which is taxing in places - but not for long!
I think there are 5 options:
Walk the 29 kms. Take your time, leave as early as you can. Carry food. There is water along the way.
or
Check that the Bar Julia will be open (telf. 981630242) and when you reach there arrange for a taxi to take you to Hospital de Bruma which is only 10 kms or so away and you may decide to be taken back in the morning.
or
Arrange for Antonio from Meson do Vento to come and pick you up. This is a service he announced when I was there and he had produced leaflets. However when Sil from the Forum tried to arrange this he went to the wrong bar Julia! Hopefully it is all sorted out now but you should make sure if you e mail him to arrange this. He speaks english. Make sure that he knows to pick you up at the Bar Julia, San Paio de Vilacoba, telf. 981630242
or
Before you leave Betanzos arrange for a taxi to pick you up at the Bar Julia (for example if it is closed) and either take you forward to Hospital de Bruma or back to Betanzos to sleep and go back the next morning. The staff of the Tourist Office in Betanzos are very helpful and will assist with this.
or
Use the route which TiaValeria and Terry intend to take which is to miss out this section and to go via Carral where there is accomodation. I am sure they will tell you more of their plans. The Tourist Office in Betanzos will give you a map of the area which I am sitting here looking at and it is perfectly feasible. Google maps will also help.
Of all of these my own preference would still be to walk the Camino route and go over the hill to Bruma even if it has to be done in two halves.
2 Sellos
Sellos are available all along the route. The Guide mentions a lots of places with sellos - tourist offices, cafes, churches and so on. Get a sello wherever you sleep and make sure you get one in Ferrol. You will have no difficulty with sellos.
3 Hills
Most mornings you will do a little cardiovascular exercise as you walk out of places
If you leave early and take your time this will present no difficulty. The Camino Ingles isn't a difficult route but it does have a little bite from time to time. My advice is to breakfast well and to carry some energy food - chocolate, dried fruit etc - you won't need a lot!
4 Blisters
If you prepare well by walking as often as you can and perhaps go for a full day hike or two before you go
AND you treat your feet well you should be fine. There is a lot of advice here on avoiding blisters. If I were you I wouldn't take the risk of discovering "what is best for you" I'd simply follow the recieved wisdom by covering your feet in vaseline (or similar) at least once per day and then wear a thin pair of liner socks and a thicker pair of walking socks. There and places all along the way where you will be able to take your boots or shoes off to rest or bath your feet in the cold water of Galicia. Lovely
If a spot on your feet becomes uncomfortable stop immediately and deal with it. Often socks off and back on again works fine. In the event you do get a blister my own preference is simply to clean the area and pass a sterilised threaded needle through the blister leaving the thread in situ to drain the blister. Cover with a plaster and all will be well.
Going on pilgrimage is a big thing so you are right to ask whatever questions you have - don't hesitate.
If there is time later in the year I'll try to get out and explore the Carral alternative to write it up for the guide if someone doesn't volunteer to do this sooner! - but the Betanzos to Bruma stage is very rewarding - you'll be proud of yourselves!
Best wishes
John