wallpaper steamer

Hoping for advice. I've used a wallpaper steamer to remove a border that has been up probably twenty odd year and it has caused patching when I've scraped the paper off. Please see photos. How can I rectify this? Also, there is a strip of paper in the corners. What is this? How can I fix it?

I've been trying to steam off wallpaper in our 1920s semi we've just moved into. In the kitchen there's a small alcove which is like to restore as a pantry but having steamed off the textured wallpaper and revealed a grey plaster underneath I've realised it's been wallpapered over to obviously...