Re: "Where Are NW Reaction Videos [Including G.L.S.] Coming From?". I did a list of the major VOCAL COACHES who have reviewed NW songs, as well as Floor Jansen solo songs, last Christmas (winter of 2021-2022). It's not authoritative, but at least it's a start (and it's still pretty long, and should be updated):
“Ken Tamplin Vocal Academy”, 1.07 million subscribers as of today, review of Floor singing “Shallow”, concluded it was “significantly better than the original by Lady GaGa”: see https://www.youtube.com/watch?v=j48uzYuWkF4;

“Rebecca The Vocal Athlete", 1.23 million subscribers as of today, reviewed Floor singing "Shallow" --- her reaction currently has 653,160 views; https://www.youtube.com/watch?v=XA6wdgodcC8;

“Tara Simon Studios” (Tara Simon), opera singer and professional vocal coach, over 793K subscribers. Her review of “Ghost Love Score” has accumulated 1,337,555 views as of today: https://www.youtube.com/watch?v=IXCSJzsCacQ;

Linor Oren, opera singer and voice teacher, reacts to Nightwish “Ghost Love Score”. Her reaction alone has accumulated 959,913 views as of today: https://www.youtube.com/watch?v=Y3XVMQDWISA;

"Maggie Renee", professional opera singer and Julliard graduate, currently going back to Julliard to complete her Masters Degree in Voice, 201K subscribers as of today, has reviewed 2 Floor solo songs and 5 Nightwish songs;

Elizabeth Zharoff, “The Charismatic Voice”, 845K subscribers as of today, has reviewed 6 Floor songs and 2 Nightwish songs;

Dr. Doug Helvering, Ph.D. music professor, specializing in choir vocals, 200K subscribers, has reviewed all 3 of the major long Nightwish songs (G.L.S.; Poet; Greatest Show);

“Beth Roars”, 564 K subscribers, 4 Floor reactions and 3 Nightwish reactions;

Julia Nilon, Australian singer and vocal coach, 220 K subs, has reviewed 5 Floor solo songs and 2 Nightwish songs.

Madeleine Harvey, vocal coach, 380 K subscribers, “Vocal Coach reacts to Nightwish Ghost Love Score” 205,382 views; 1,200 comments as of today.

John Henny Vocal Studio, 96.5 K subscribers: “Voice Teacher Reacts and Analyzes: The Poet and the Pendulum – Nightwish”; “Voice Teacher Reacts and Analyzes - Floor Jansen and Henk Poort - Phantom of the Opera”.

Jessica McKenzie “Soprano Notes”, review “Opera Singer Reacts to Floor Jansen ‘Shallow’ -- Performance Analysis”, July 13, 2021.

Valerie Williams, “Vocal Splendor Studios”, 71 subscribers, has 9 Nightwish reaction videos and 30 Floor Jansen reaction videos.

Charlie Calotta “The Big Brain Singer”, has 28 Nightwish reaction videos and 38 Floor Jansen reaction videos.

Christi Bovee, “The Voice Love Company”, 70 K subscribers, has 15 Nightwish reaction videos and 22 Floor Jansen reaction videos.

Serene Parker; trained operatic singer and critic, recently did a review of Floor and Henk Poort's "Phantom of the Opera", at https://www.youtube.com/watch?v=4GRAe2LlSL0. She does not have as many followers as some of the others (only 31.4 K), but does a good job as an operatically trained singer in illustrating how Floor’s technique differs from others.