I think she did believe him, but it was too late for her. She was already ill unto death, and finding out he was already dead, she wanted to die and be with him.

She likely believed him yes but didn't care much at that point, he had already broken her heart and destroyed her. Remember what Merteuil tells Valmont after he informs her that he broke with Tourvel in the most heartless way possible, and wonders if he can possibly win her heart back; "When one woman strikes at the heart of another she seldom misses, and the wound is invariably fatal". The damage was done, Tourvel didn't care much anymore about whether Valmont loved her or not, he'd already destroyed her; to the information that Valmont truly loved her she simply says "Enough, draw the curtains".

yes but he is known for playing games with people. Working on their emotion just so can feel the power that he CAN do it to them, having them doubt everything they believe in and seeing them betray what is most important to them. Why stop when he is dying?? This could be another sick game of him. Him having the last laugh.
I personnally believe he really loved her, but i don't think she believed it anymore. She was hurt too deeply. she doesn't wanna take a chance of believing him a second time and again betraying her marriage. I think she feels that she is punished for betraying her marriage. She even says so to Mme de Volanges: 'i am dying because i wouldn't believe you".
So why stop believing Mme de Volanges now? Look what trouble it has brought her. Remember she was religious. So she must feel she has betrayed God.

Of course she believed him. Those were his last words, and there were no reasons to lie anymore.
Still pond / A frog jumps in / Plop!