Who are "Yir'ei Shemi"?
Nedarim 8b: They are people who are afraid to say Hashem's Name in vain.
Malbim: They are Tzadikim who served amidst love.
What do we learn from "Shemesh Tzedakah u'Marpei"?
Nedarim 8b #1: Dust that can be seen (only) in the sun heals.
Rashi citing Nedarim 8b #2: In the future Hashem will remove the sun from its case. It will heal Tzadikim.
Radak: The future day [of judgment] will consume Resha'im, but Tzadikim will be saved from all evil, and rejoice.
Malbim: The good deeds that Tzadikim (who served amidst love) did are like a shining sun. There are two aspects of Divine light - light for Tzadikim and fire for Resha'im - "v'Hayah Ohr Yisrael l'Esh" (Yeshayah 10:17), like I explained there.
Why does it say "bi'Chnafeha"?
Radak: The sun will spread its light over the land. Its shining will be like spread wings. And so it says "k'Shachar Parus Al he'Harim" (Yo'el 2:2).
What do we learn from "[vi'Ytzasem] u'Fishtem k'Eglei Marbek"?
Nedarim 8b: In the future Hashem will remove the sun from its case. Tzadikim will delight in it.
Rashi: Pishtem is an expression of fatness, like "Ki Safushu k'Eglei Dashah" (Yirmeyahu 50:11). Marbek is [a calf] entered into the stall to fatten it.
Radak: Wherever you go out, you will increase, like calves being fattened. They increase fat and meat. Pishtem is an expression of increase, like "Ki Safushu k'Eglei Dashah", "u'Fasu Parashav" (Chabakuk 1:8). Its grammatical form is like v'Kamtem, just here it has a Chirik. It should have said vi'Yfishtem, like "vi'Yrishtem Osah (Devarim 11:31). Since the middle letter of the root is omitted, its vowel (Chirik) is moved to the first letter.
Malbim: You will spread like calves brought in to eat and thresh with their feet.
