I agree with your last point entirely, but unfortunately it's a politically unacceptable one. Passing some sort of civics exam before having the right to vote would be fine by me.
It's the same justification given for not letting minors vote.
If it's acceptable to de-juris disenfranchise the under-18s on the spurious grounds that they are incapable of voting effectively, surely we can de-facto disenfranchise any adults incapable of operating an AV ballot paper. (which only requires the ability to count up to 1)
