Licensing is probably the biggest headache I’d guess - being insolvent the airline won’t have a valid license to operate into those countries any more and civil

Licensing is probably the biggest headache I’d guess - being insolvent the airline won’t have a valid license to operate into those countries any more and civil aviation regulators are usually very strict on that sort of thing. But as you suggest, insurance and ability to pay landing and fuel charges (sometimes aircraft are impounded by airports for failure of the owners to pay these charges) are some potential issues too. Then the planes themselves may also be leased and so the owners of those planes would want paying too, possibly including any arrears, before allowing them to be used. None of these are obstacles which couldn’t be overcome but it’s quicker just to use someone else who doesn’t have those issues.