I think pattern and his kind have been there with Shallan since she killed her mother. The light in the safe was in her head, a lie she told her self.
When pattern gets Andolin to reserve a room for her and tricks/gets her to recreate the scene when she killed her mom:
"A painting had appeared, part of the illusion, and it was outlined with a white glow. Something was hidden behind it...."
"...she pulled aside the picture, revealing a strongbox in the wall beyond. She raised a key, and hesitated. "Mother's soul is inside." "Mmm... no. Not her soul. That which took her soul." Shallan unlocked the safe, then tugged it open, revealing the contents. A small Shardblade..."
"...Father took you from me," Shallan said, "and tried to hide you in here. Of course, that was useless. You vanished as soon as he closed the strongbox..."
Shallan believes that her and Lin were not thinking clearly, that is why he tried to hide a Shardblade. Of course he probably later opened that strongbox and saw that the blade was not there. Shallan probably kept seeing the glow out of guilt, as some kind of reminder of what she had done in spite of her suppressed memories. Shallan is very good a lying to herself and not seeing or hearing what she doesn't want to see or hear. I guess being around Jasnah, wanting to soulcast made it easier for pattern/lie spren get through her denial?
The Ghostbloods seem pretty good at what they do, I don't think getting a working soulcaster and training/gettings someone to use it is beyond them. It seems more likely to me than trapping pattern to make there own soulcaster.