Re: removing dried glue on wallboard
It's just not going to happen. Not only will you not be able to remove all of the glue, you won't be able to do it without some level of damage. What you need to do is just use care and caution during the removal process to minimize the damage, then the walls will be skim coated with drywall compound until smooth. From here, you can decide whether add texture or leave the walls smooth. It would be best to prime then paint the walls, even if you plan to install wall paper.
As far as tools required, a 6" putty knife will probably be the fastest, however the smaller the implement the more control which equals less damage. Alternatives would be to use a sc****r tool such as this:
You can sharpen the edge as necessary with a belt sander or hand file.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!