Could Madoka ever accumulate despair, or does her Soul Gem stay pure forever? According to Kyubey, she takes the despair away from magical girls about to become witches. If her Soul Gem became tainted, she could potentially become a witch and destroy herself (through her wish). Would that even be possible??
she could potentially become a witch and destroy herself (through her wish). Would that even be possible??
yes, we see this happen in Episode 12 after Madoka makes her wish that ultimately leads her to become a Goddess.
After Walpurgisnacht is destroyed Homura finds herself on the moon watching Madoka's tainted Soul Gem streak across the galaxy, collapsing into a Grief Seed before becoming Kriemhild Gretchen. it is then that Madoka appears and we see her assume her Goddess form and as per her wish she can destroy even her own witch form. however Madoka must still exists to continue erasing witches even after she destroys herself and this paradox is what lead to the Universe having to be recreated.
in Madoka's Universe however we can't confirm if Madoka even has a Soul Gem. while amiami has had an Ultimate Madoka Soul Gem on sale once before (historical images) the problem is that Madoka's Goddess form doesn't show anything that looks like an opal on her in her Universe as shown in the Rebellion movie
and Magical Girls generally have the same coloured gem on their body as their Soul Gem and Ring. what's more Ultimate Madoka's gems on the upper part of her dress might be the same colour and shape as her Soul Gem in the previous universe but it does not show a noticeable metal frame around it like in her Magical Girl form (Designs for Homura, Kyoko, Sayaka, Mami, Nagisa showing their Soul Gems in Magical Girl form).
there's also the fact that it was the Incubator's logic that a Magical Girl's Soul exists outside of their body in the form of the Soul Gem. it's possible that after becoming a Goddess Madoka's Soul is back in her own body.
i would speculate that in her universe Madoka, as a Goddess and the concept called the "Law of Cycles" that's always existed in the universe, is a difference existence than a Magical Girl and not bound by their rules. even her own wish states this
I want to erase all witches before they are even born. I will erase every single witch in every universe, past and future with my own hands...I don't care what you call it. All those magical girls who held onto their hopes and fought against witches I don't want to see them cry. I want them to stay smiling until the end. If any rule or law stands in my way I will destroy it. I will rewrite it. That is my prayer. That is my wish. Now grant it, Incubator!
and as in her universe the Law of Cycles can not become a witch
As Madoka herself explains at the end of episode 12 of the anime,
if her wish truly came true then even she would not become a witch which is why she became a god and was incorporated into the Law of Cycles. She is even shown stopping her own witch form which is why she was able to create a new world in which witches didn't exist. So to put it bluntly, the answer is no.