On your first point: you may well be right, but it still doesn't explain why she made no mention whatsoever of the possibility at all. If she said at any point "There's a chance that Littelfinger may be heading directly to Winterfell with a large body of cavalry." Jon could have sent riders south to confirm (or refute) the presence of the Vale army and adjusted his plan to suit.
As for your point in bold: I think several factors point to this possibility. Firstly it's been signposted in earlier episodes, with both Brienne and Littelfinger making points to Sansa about how she cannot trust Jon because he isn't a full blood relation. Littlefinger urged her to seize Winterfell with the Vale army, loyal to her, rather than letting Jon do it with the Willing army loyal to him. Secondly, she simply didn't seem at all fazed by the likelihood of Jon's death in the battle. When the camera settled on her as she crested the hill with Littlefinger, as the Vale knights charged, she looked as if she felt contempt for Jon's tactical stupidity, but there was no flicker of concern. Thirdly, the Vale arrived just in time to defeat Ramsay... but not until the bulk of Jon's army had been chopped to pieces. Maybe this was deliberate? It would go some way to explaining Sansa keeping the Vale army a total secret from Jon.
I don't know, but I think she may have turned a corner in terms of who she is prepared to trust and rely upon. Maybe she's decided to trust nobody and just rely on herself... Jon seems to be an emotionally-driven man who is a poor battlefield commander, so it's not impossible that Sansa would rather have her own castle and her own army than rely on Jon to protect her and treat her well?