1)
What is the tense of "Hiv'ishu"?
1.
Radak: It is Hif'il, but it is Po'el Omed (the action applies only to the subject; my abscesses themselves rotted). The same applies to "v'Lo Hiv'ish" (Shemos 16:24).
2)
What are "Chaburosai"?
1.
Radak: They are my abscesses full of pus. The pus was inside a long time, so the skin over it melted, and the pus came out and decayed.
3)
What is "Ivalti"?
1.
Radak: (My folly.) I did not guard Hashem's way.