At least here in Germany, the Point "It's a donation, not a payment! No legal issues!" would be a good point because there really won't be any legal issues.
Since you can get the product (in this case, the mod) completely for free and every cent you give is completely optional, it's a donation and therefor no problem with taxes.
And the "magical category" would be donation, it is it's own category here.
Your comparison with the " "Limited Liability Company" fails in this regard, as it's something completely different.