"If you withhold the gifts of the Kohen, by your life, you will need to come to him to bring him the sotah."
The Kli Yakar analyzes the connection between stiffing the Kohanim and one's wife going astray, quoting sources from Gemara Sotah and Mishlei, that describe how associating with "loose women" brings a man to poverty.
And when the wife sees that her husband comes to poverty, she suspects of him that he came to poverty because he has been associating with "loose women." .... When she sees that he suddenly loses all his wealth, which is not the normal way of things, then for this reason, she, too, behaves licentiously after him [i.e., the wife follows in the adulterous footsteps of her husband].
Therefore, it says "and deals treacherously with him," meaning that if his wife shall go astray and deal treacherously with him by means of this betrayal, then this betrayal hinges on him because he caused her, out of her suspicion, to want to behave like him.
And all this was caused because he dealt treacherously with the holy things belonging to the Kohen, by not giving the Kohen his holy things. For through this, he comes to poverty. And she suspects him of associating with "loose women," [an act which] destroys wealth.
And the truth is clarified through the "Water Test" because Chazal said (Sotah 28a) "and a man will be cleansed from sin. If he is innocent from sin, the water tests her, etc." - meaning, [innocent] from this sin, which resembles her sin. If he is innocent of what his wife suspected him, then the verdict [דין] is that the waters will test her.
But if he is not innocent of this sin and her suspicion of him is the truth, then there is no verdict [דין] that the waters will test her after he himself has been associating with "loose women."
So she has a bit of an excuse [ויש לה קצת התנצלות].
And regarding that which it says תשטה [she shall go astray]: It is a term [derived from] the word שטות [foolishness]. Even though every sinner is call a chump [כסיל] and a שוטה [fool], nonetheless, it [uses this specific term] to imply that she is davka in a state of mindlessness [הומיה פתיות] as she doesn't know what caused his poverty, and she should have understood that it was the withholding of the gifts that caused his poverty.
The word פָּנָיו translated as "His Countenance" can also mean "His Face."
The Kli Yakar takes this opportunity to describe the act of hastarat Panim, which literally means "the hiding of the Face." It is a terrifying state in which the Jewish people have sinned beyond all limits and ignored all messages to do teshuvah. Therefore, Hashem hides His Face, so to speak, so the world acts toward the Jews as if Hashem is no longer watching them, may He have mercy.
Obviously, Hashem does not have a physical face and this is all metaphorical. So the Kli Yakar poetically describes a bit more of what is going on and why this blessing is so important and powerful:
We see from this that even in an hour of wrath, during which Yisrael deserves to be in a state of hastarat Panim, and every person who hides his face looks as if he is facing downward, so to speak, that nonetheless The Holy One Blessed Be He says it is the way of acquiescence [ויתור], I will raise to you My Countenance so it will be directed across from your face because "raise" is only regarding that which is liable to fall or to face downward.
This is my own translation and any errors are also mine.