The RCC is pretty rigid on divorce/conversion issues. My brother in law began the conversion process, and midway thru was told that the RCC still recognized his first marriage, not his then-current marriage, and that he’d have to get an annulment to continue thru to conversion. No way was that going to happy. Wife #1 would never have given an annulment; Wife #2 conveniently died within a month. Otherwise, I don’t think they’d have let him convert and/or take sacraments.
An ex-spouse's permission is not required for a decree of nullity, and even their cooperation in the process is not an absolute requirement (but it is helpful).