Background: Character of approx 9th level is attempting to find something hidden in the Negative Plane. They will not have access to spells of 3rd level and higher, let alone 7th level and higher that allow for planar travel to escape the plane and so my main concern is the bag of holding effect created when placed inside an extradimensional space. However, I have included a partial description of the Astral Projection spell for two reasons, 1) because similar to the bag of holding effect it leads to the Astral Plane (albeit with the silver cord at first) and 2) to make the question more general to characters with access to higher level spells.
In MToF pg 216 included in the description of Nightwalkers is the following:
Mighty Spawn: One can reach the Negative Plane from the Shadowfell, much in the same way that it is possible to step from the Material Plane into the Shadowfell in a place where the barrier between planes is thin. Stepping into the Negative Plane is tantamount to suicide, since the plane sucks the life and soul from such audacious creatures and annihilates them at once. Those few who survive the effort do so by sheer luck or harnessing some rare form of magic that protects against the hostile atmosphere. They soon discover, however, that they can’t leave as easily as they arrived. For each creature that enters the plane, a nightwalker is released to take its place. In order for a trapped creature to escape, the released nightwalker must be lured back to the Negative Plane by offerings of life for it to devour. If the nightwalker is destroyed, the trapped creature has no hope of escape.
The description of the Bag of Holding in DMG pg 154 includes the following:
Placing a bag of holding inside an extradimensional space created by a Heward’s handy haversack, portable hole, or similar item instantly destroys both items and opens a gate to the Astral Plane. The gate originates where the one item was placed inside the other. Any creature within 10 feet of the gate is sucked through it to a random location on the Astral Plane. The gate then closes. The gate is one way and can’t be reopened.
The Astral Projection spell in PHB pg 215 indicates the following:
You and up to eight creatures within range project your astral bodies into the Astral Plane (the spell fails and the casting is wasted if you are already on that plane).
later in the same description:
Your astral form can freely travel through the Astral Plane and can pass through portals there leading to any other plane. If you enter a new plane or return to the plane you were on when casting this spell, your body and possessions are transported along the silver cord, allowing you to re-enter your body as you enter the new plane.
(NB: I am ignoring that the Astral Plane does not have portals to any plane, rather just the Outer Planes, Material Plane, and Ethereal Plane as getting to the echo or inner planes is of secondary concern)
I understand that Gate and Planeshift spells can also be used to travel to other planes and would apply. I do not believe the Etherealness spell would apply, as while the Negative Plane clearly borders the Shadowfell and the Shadowfell borders the Ethereal, it is not clear to me that it can be concluded that the Negative Plane borders the Ethereal.
Accordingly, can a character/creature escape the Negative Plane without its Nightwalker being lured back to that plane?