Interesting. To me, a form of Iselinde sounds most plausible. Is Iselinde used at all in Germany nowadays?
Isengard makes me think of The Lord of the Ring.
"You sought a flower and found a fruit. You sought a spring and found a sea. You sought a woman and found a soul. You are disappointed."
"It does not become me to make myself smaller than I am." (
Edith Södergran 1891-1923)