@Mrna , welcome to the forum.
1/ Credencial: Readily available at the Oficina Municipal de Turismo e de Atención ao Peregrino at the Port (starting point), according to Google it's open 8 until 13.00 daily.
2/ in April '23 I just carried a silk liner, but I sleep warm so each to their own.
3/ I learnt a few basic phrases and words of Spanish, I didn't really need it but it was nice/ respectful. Because so many fellow pilgrims were Spanish I sincerely wish I had learnt more. Most cultures love it when you make an effort, I'm sure any attempts you make will be welcomed!
4/ most people complete the Inglés in four to five days, after about eight weeks of moderate training I easily walked it in five. I didn't find it in the slightest bit 'rigorous' but then I've done a moderate amount of hiking when I was younger so my expectations are different from somebody that hasn't.
Some prefer to take eight days or even longer.
5/ because of time constraints I pre-booked almost everything. Normally it would be unnecessary however it was Semana Santa which is extremely busy. The Camino was absolutely packed, but with the exception of Bruma there was accommodation available literally everywhere. Often the albergues would be completely full, however there was always a hotel.
The only real pinch point on the entire route is Bruma. Especially if there is a public holiday or long weekend of some description, because on those occasions the Spanish flock to this Camino. Accommodation is very limited there, both the public and private Albergues are small, and there is just one pension and a hotel nearby (1.5km away). On multiple occasions last year to my personal knowledge everything was full and people either had to taxi or walk on an additional 10 kilometers. Something to keep in mind!
It's a wonderful first Camino, enjoy!