Serack
My take: When Lash rebelled against her originator, the bit of lasciel's energy that was giving Harry access to hellfire was either destroyed of fundamentally changed to the point where it was no longer the same thing... I.E. Soulfire and Hellfire are 2 sides of the same coin and that coin had now flipped. So yah he wouldn't have access to hell fire any more (He didn't obviously) and instead he had access to soulfire, he just didn't know it/didn't know how to access it.
The problem I have with this statement is that you are essentially suggesting that a mere imprint of a time immortal being was able to give Harry access to Soulfire. The reason I think that Lash allowed Harry the use of Hellfire is because that is the nature of the Denarians. They offer power, and Lasciel herself is known as
The Temptress. If that is her nature, then offering a small sampling of power in exchange for binding his soul is well within her nature, thus the imprint is allowed to give access to Hellfire.
Harry gave Lash, a mere shadow free will, to go against her creator, but to say that the mere shadow that was Lash gave Harry access to Soulfire, and allowed him to use it after she was no longer present within him as far as we know, I find that to be an extreme stretch.
Furthermore, in Turn Coat, Uriel himself converses with Harry, informing him of the good side actually doing something, just not in a noticeable way at first, then Mab appears and gives us a little backstory regarding Uriel. If Lash gave Harry the ability to use Soulfire, then you are essentially saying that Jim Butcher himself planted Uriel, a freaking Archangel, in the series as a red herring. Possible? Yes. But I disagree.