From a writing perspective, I think that the scenario explained in Blood Rites where Thomas was able to pull away at the last minute is probably the truth as well as the birth of the true love protection between them. Faking true love seems like it would cheapen their character arcs in an unnecessary way (e.g. if it wasn't actual love for Justine, what motivates Thomas in PT?)
As far as the timeline for Justine's infection, I think that's a topic worth delving into more deeply. Assuming that the exact point Nemesis infection has some clues in the text (since other major infectees were; Lea, Maeve, Cat Sith), we can start narrowing things down.
Beside claims that it had Justine since she had become close to Lara.
"How long?" I asked. "How long have you been in Justine?"
Justine waved the steel bar in a vague gesture. "Mortal time is such a limited concept. A few years. Ever since she became close to Lara."
Going off this, it seems like it would have to be at least a significant period after Blood Rites. In White Night, Justine is still playing ditsy, but she's clearly Lara's top aide. Significantly, White Night also shows us the Black Council interfering in the White Court. Cowl got a personal view of Justine blowing her air horn and being protected by Lara, not to mention whatever information Madrigal passed along. I'd imagine that that at least would have put her on the Black Council's radar, hence a Nemesis interest.
Justine was a small part of Turn Coat where she was in the club Zero and burns Madeline Raith. I don't see an obvious time for her to be taken here.
Justine's next big appearance would have been in Even Hand, about eight months before Changes. There, Lara sent Justine as a messenger to the Fomor but she was abducted by a different Fomor lord, Mag. While in Mag's custody, Justine managed to free a bunch of his prisoners before fleeing to Marcone for protection. This could very well be where she was infected.
Justine was also a player in the Bombshells short story a bit before Ghost Story where Molly prevents the Fomor from bombing the svartalves at a treaty signing. Justine being infected in Even Hand makes her involvement here need some explaining, but that's probably doable.
After that, Justine was a hostage of the Redcap in Cold Days. This might be an opportunity to get to her, but we learned in BG that the Redcap was Mab's eyes on Maeve in CD. It would have taken more than just the Redcap for that to be the infection time. Plus, it doesn't exactly fit the "ever since she got close to Justine" line from Nemesis.
Finally, we know that Justine was infected before the start of PT since her getting pregnant was Nemesis's idea. Of all of Justine's on-screen time, Even Hand seems like the most likely suspect from what we know now.