if you hold a steamer in one place for long, the plaster will be overheated and blow.
Woodchip is difficult to get wet enough to soften the paste. because it is usually painted and the paint stops water penetrating.
An ordinary wide metal scraper will often knock the tops off enough of the bits for water to start getting in. I prefer to use a garden sprayer, with a tiny bit of WUL in the water, and mist it again, and again, and again - never letting it dry out as you go round and round the room, and it gradually soaks in. No benefit in applying so much water that it runs down the walls onto the floor.
If you start scraping at the top, then next time you spray, some of the water will run doen behind the paper. Once you've managed to start that happening, it gets easier and easier.
If you are intending to paint the wall, you have to get all the paste off. I find this is easiest again using the sprayer and the wide metal scraper. The paste will come off like slime which you wipe off on your sleeve a rag and wet and scrape agin until no more comes off.
Don't put PVA on any wall that you hope one day to paint.