Ziddina, a few clarifications, because I think you misunderstood some points. Or at least this is what I understood from the movie.
[SPOILERS coming]
Sara's mother was a conscious JW, she was only in to be able to see her children, and maybe to help them get out. That's why Sara confided in her. It was not the mother who rattled her out, she protected Sara's secret, she even said to Sara's sister that Sara is indeed living with her (although it was obvious she was not).
For the same reason was Jonas returning. In every scene he's portraying a truly lost boy, who just can't go on alone like that. His decision to return came before Sara completely cutting off all ties.
After their first intercourse Sara never went in nun mode. She slept with Teis, she just kept her stuff in the basement and said "in case someone asks, I sleep here". When the elders came to visit, Teis rushed to bring all her stuff down from their common bedroom. It was pretty clear to me they continued having sex. Sara wanted to avoid the shunning, that's why she tried to keep all the appearences.
I do agree with her having a great support system. However I do admire her (the real girl, if she did something similar) because of her moral standards that is way higher above the average JW's. She realized it's not fair for leech off of well-meaning people in her road to recovery, and that her love for Teis was stained by the emotional damage the WT did. That's why she said "I have tried to replace all that I lost with you". Her love might not have been true because of the circumstances, and wouldn't have been fair to Teis to keep it up in hopes that it will get better in time.
True, she was sending mixed messages, and the whole thing was just bizarre, but that's the price you pay for getting involved in a high-control cult.