Neil would never doubt Avni, he knows his wife too well and the only reason he did this is because he knew she was hiding something from him. Neil knows when Avni becomes Ananya, he can see through her and probably knows her better than she knows herself. The questions Neil asked proved he is aware of why Avni would hide something from him, is she being threatened? Is she trying to save someone? It was perfect! I really like how they had Neil learn the truth.
Yes Vidyut always planned to tell Neil the truth about Mishti being his. Vidyut was never going to let Juhi have her happily ever after or have Neil raising his daughter, he just wanted the pleasure of seeing his opponents downfall.
About the IF article lets not forget they don't have the best track record when it comes to spoilers for this show. Remember they said Juhi would be mahaan and get shot during the RM track concluding? Next there was Vidyut hiring Neil to work at the camp and Mishti not accepting Neil. Now there is this. I don't think there will be a leap but I do believe there will be some truth to Avni being set up by Vidyut and Juhi meeting her end. Vidyut wants revenge as seen in the precap and I wouldn't be surprised if he sees through the trap set by Neil and Avni for Juhi. Vidyut could very easily kill off Juhi who is off no use to him after its known Mishti is his and he could have Avni take the fall. You just never know when it comes to NK.