RTFM people. HoW made some changes to opposition schools. It's not unusual to have two oppositions schools at all. That's why specialist mages suck!
If you want to have it easier, don't install HOW and the "revised oppositions schools" will not take effect?
Also, if you make a high level specialist mage in IWD and copy spells into his book that would be considered "opposition" under HOW rules; then install HOW and import the mage, he will still be able to cast the spells he has written, even if they are of an oppositional school.
BTW, what kind of mage "rarely uses" illusion spells?! what about mirror image? that's the bread and butter of a combat mage's defence!