Will the one being supplicated for be held accountable for her bad whispers and thoughts that entail supplicating against others with evil, and will this type of supplication be accepted?

Since you know that these are satanic whispers (waswas), strive to repel and fight them. Do not allow Satan to take hold of your heart. Instead, turn away from them and pay them no attention. These whispers do not harm you, nor do they harm those about whom you experience these whispers. Remember that Allah has forgiven His servants for what their souls whisper to them, as long as they do not act upon it or speak about it. Attach your heart to Allah, trust in Him, and strive to supplicate Him to remove these whispers from you. Fight them as much as you can, and you are hoped to be rewarded for your struggle against them.