Umm we will never come to know abt what actually happened at that time but My heart goes with the RCM story because Ahilya was one of the five major Mahasatis of our Mythology ever known and I hold deep respect and sympathy for her from inside, Imagining that she had done the union with Indra willingly in completely undigestable for me!
Some of the Versions even say that Gautam rishi actually never cursed her but only rejected her as his wife, Ahilya herself got hurt by Indra's treachery and her husband's rejection that she became stone cold. And remained like that forever till Ram's feet didnt touch and purified her!
So according to this version also she was deep guilty for what indra had done to her without her knowing!