Hmm...never heard of iso "ensrusting" while isomerizing. You should distill and see what temperature it comes over at. Water contaminated iso should not do that. You need to distill and see what comes over.
Read Post 306186 (https://www.thevespiary.org/talk/index.php?topic=8828.msg30618600#msg30618600)
(abacus: "Easy iso", Methods Discourse)
Its pretty much idiot proof if you follow the directions.
Also, your temp is too hot. For iso-safrole you add 1.5% by weight dry KOH to safrole, and heat to 160C, hold it there for 12hours under good stirring, then distill to get back ~95% as isosafrole. Easy as that.
Also to clean out tars from a flask, acetone ALWAYS works excellently.