Very interesting finds. I think the fact they have gone mad AND they are undead and they are still using their druid powers seems like definitive proof that one can be undead AND be a druid. And this stuff is 20 years old. As for stuff like them using non druid spells, remember our abilities while canon powers our classes might have are ultimately abstractions for a video game too.
There’s no reason a warlock couldn’t tap into the void and use shadow priest abilities like mind control or mind flay if they wanted to learn those powers for example. So the fact some druids are using chain lightning and lightning cloud doesn’t mean much. In fact I find it very likely the devs back then were inspired by druid myths as well as stuff like Dungeons and Dragons druids. 3.5 druid spells definitely had lightning spells like that.