I'm not aware of any good EU nickel producer, there are some...

I'm not aware of any good EU nickel producer, there are some deposits in Finland...

Closest to EU and "best" Ni stock could very well be Nornickel (Norilsk Nickel) which mines in Siberia and has several JVs with factories / battery companies in Europe. It is a completely absurd company, they basically built an entire company-owned city in the middle of nowhere (https://www.reddit.com/r/UrbanHell/comments/7zxhu3/the_most_depressing_city_in_russia_norilsk/) with mainly airplane access. Most of their revenue right now is from palladium but they also produce nickel, copper, platinum, cobalt and various rare metals (ruthenium, iridium...) from the same resource.

It's a huge company with 80000+ employees so you can't expect x-bagger like with $EMN but at least they pay nice dividends. The bad thing is that the mineral extraction tax in Russia is planned to be increased by 350% from 2021 and Nornickel will be one of the top three most hit companies there.

The resource is so big that they have negative production cost of nickel, after credits. Their production "cost" of nickel is more than MINUS $10k / tonne because of prices of other products (palladium, platinum, copper...). Their factory also caused a massive fuel leak which polluted a large river in Siberia this year; the company is partly owned by two Russian oligarchs which keep each other in a deadlock, so the governance isn't that bad for Russian standards.

This would be unimaginable in Europe and I don't know any Ni mine in the region.

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Back to manganese - I know what was announced at the Tesla battery day and I'm sceptical about Elon's big plans and announcements. China and Europe will probably have a much bigger word in this and I don't know what they will prefer (only thing I'd consider certain is to reduce cobalt content).