First of all, thank you for rescuing these kitties!
Here is a great link to information on socializing feral kitties: http://straypetadvocacy.org/html/soc...feral_cat.html
Cats are territory-oriented. Right now, the corner is their territory, though they're making the room theirs. They don't understand yet that they can trust you - and that's something that only comes with time, and it works on the kitties' clocks, not our people clocks.
Since they're using the litter boxes, you may want to consider making one of the litter boxes a covered box. That'll help prevent the litter from flying everywhere. If they use it, make all the boxes covered boxes.
You can also consider using a rubber maid type container as a litter box - something with tall sides. That'll help too.
Make sure you don't leave any "wand" type toys out for them to play with. Little balls, furry mice, etc. are great. If you get to the point where you can interact with them with wand toys, take them with you when you leave the room. Ferals tend to chew on string (reminds them of mice tails) - and if they swallow it, it can get wrapped around their intestines and require surgery to fix.
You may also want to consider some other types of scratching implements - a scratch pad, a cardboard scratcher that lies flat on the floor. Cats love to scratch and stretch when they wake up, so placing scratchers near where they sleep is always a good idea.
Ferals are taught by their mums not to talk, so not talking is normal.
You may want to consider putting some empty boxes in the room so they've got hidey places. This will help them feel safer.
Also, don't reach out to them, try to touch them, or even look at them. If you look at them, look over their heads or at their foreheads. Looking at cats in the eyes is a sign of aggression.
Take an old t-shirt or sweatshirt or something and go jogging or biking or something to get it good and sweaty. Leave treats out for them on it. Even do this to a second shirt and put it under their food dishes. This will help them come to associate your smell with good things.
Mostly - spend as much time in the room as you can. Read out loud. Knit and talk out loud or sing. Work on your computer or laptop - just be in there without trying to interact. The idea is to get them used to your presence. To let them learn that you are no threat at all to them. There is nothing you want from them. They need to learn they are safe with you.
And ignoring them - but being in there with them - is the quickest way to do this.
It can be difficult and frustrating - but that's what we're here for! Write to vent, write to ask questions - but know that with time and patience, anyone can socialize a feral that is less than a few years old. The older they are, the longer it tends to take. If they've had bad experiences with people, that will affect the amount of time it takes.
But they have no idea that petting is wonderful - let alone who they heck you are and what you want from them. With time, they'll figure out that you just want to love them and that love is good.
The most incredible thing, however, is that bond you have with a feral once he/she/they do come to trust you. And it WILL happen!