The theme of communication is a central one to this novel. As your post suggests, communication has its positives and its negatives. On the positive side, good communication fosters a tighter relationship between people; it can foster truth; it makes life more clear because a person "knows where they stand" with others. On the negative side, good communication can bring out truth that one might rather not know. These are all relevant to a discussion of the theme of communication in Ethan Frome.
In your first example above, Zeena is clearly communicating to Ethan that she notices his attraction to Mattie, as evidenced by his shaving and paying more attention to his grooming. This lets Ethan know where Zeena is on the topic of Mattie. It increases Ethan's guilt over his feelings for Mattie.
In your second example, Ethan chooses to communicate with a lie, to spare Mattie's feelings. This is in an effort to keep her close to him. This type of communication though always carries the price-tag of guilt and concern over the eventual revelation of the truth.
In your third example there are a couple types of communication going on. The unspoken communication of Ethan and Mattie "playing house" bonds their relationship. They don't actually talk about what they are doing because that would break the spell. They don't actually talk about their feelings because that would make them more real and then they would have to deal with the potential aftermath of all that. The surface conversations are light and companionable, bonding them in that way.
In your last example, it is the unspoken communication that is the tension that Powell senses. Zeena resents the threat that is Mattie; Ethan is torn between his obligations and his dreams; Mattie wants a life of stability and is uncomfortable in the current situation living in the Frome home, but has no other choices. None of these characters is willing to get it all out in the honest open, so they all walk around in the secret worlds of their own thoughts. This ends up destroying them in the end.