Need help with backdrop stain. Ideas to remove?

Truthfully, I think you are fighting a losing battle to try to remove a water stain like that, bite the bullet and replace that section.

@JerryG We use kilz cover water stains, its like a primer and it white works very well although being wallpaer you always have to worry about it coming loose.

I'd investigate the source further before repairing the wall.

In my experience, leaks don't go away on their own, they will lurk in the deep dark recesses until an in-opportune time though......

For the repair- Bin latex primer will cover most anything. Really tough stains may require Bin's Kilz Shellac. I'd say the wallpaper is a goner.

Bob

I agree with Bob @RSJB18; you have to identify the source. What’s on the other side and above the wall? Water supply, drainage pipe, sink, tub, shower, on floor above, AC ducts; or if it’s the top floor the roof.

@RSJB18 posted:

I'd investigate the source further before repairing the wall.

In my experience, leaks don't go away on their own, they will lurk in the deep dark recesses until an in-opportune time though......

Gotta' agree with this comment, I had a leak in my old house that used to show up every few months until we finally tracked it down.  One tiny leak around a vent pipe in the roof was running horizontally along the vent pipe in the attic and showing up far from the roof vent pipe, that's why we didn't find it right away.  Only when the rain came from a specific direction and with higher winds would the water come in with sufficient quantities to cause the leak to show up.