That first part makes no sense

because magnetism is the direct result of moving electrically charged particles, while as you said radioactivity isn't electricity nor is electricity the direct result of radioactivity. They aren't comparabel at all
And yes steel type would make sense tok (as it is also related to magnetism in pokemon just like electro) but llosing the ice type makes no sense since aurorus's concept of being super cold would get lost. Losing rock type would make sense as like we have seen with aerodactyl mega evolving reverts them back to how they were originally before becoming fossils.