Phew, this took some perseverance! The route through the maze traces the following overlapping path:
To navigate it, we must trace out:
a chain of character names from the books of J.R.R. Tolkien, where the last letter of one name becomes the first letter of the next.
The characters who appear are (in order):
IN > Azog, Gandalf, Frodo, Oromë, Elenwë, Elrohir, Radagast, Tulkas, Saruman, Nellas, Smaug, Gollum, Manwë, Éowyn, Nori, Is(u)ldur, Rose, Eärendil, Lotho, Ori, Ingwion, Nerdanel, Landroval, Legolas, Sam, Merry, Yavanna, Arveleg, Gríma, Argeleb, Boromir, Rorimac, Celebrindor, Romendacil, Laura, Arvedui, Imrahil, Luthien, Niënor, Rosie, Egalmoth, Hilda, Angelica, Algund, Dwalin, Nuneth > OUT
(Letters in bold here are those which must replace the question marks.)
Which means the password - spelled out by the letters hidden behind the question marks, in the order we encounter them - is:
FRIEND, which is a reference to an event in Lord of the Rings where the West-door of Moria bears an inscription with the instruction to "speak 'friend' and enter" (hence the title here!). Once Gandalf does so, the gate opens!