What is the ruling on debt that has doubled due to usurious interest, what should I do to clear myself of liability, and is it permissible for me to use the devices purchased with this loan?

Dealing with usury is a grave error, but repentance expiates it. You are only required to repay the principal amount of the loan, and you are not obligated to repay the usurious interest unless you are compelled to do so, based on the Almighty's saying: ﴿But if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged.﴾ You are permitted to benefit from what you consumed the borrowed money for, because the prohibition of the loan pertains to the liability of the borrower, not to the money itself.