You'd need to be more specific about the wording, but if it says it can only be distributed from their site then the answer is "yes". Worse still, it means that if anyone installs that snippets, they no longer have the right to distribute their mud, either.
No - a derivative must follow the parent license. For example, if you create a Diku derivative you MUST still have the Diku authors listed in the credits - you cannot decide to drop that clause just because you've created a derivative. You can be more restrictive (eg, require that your own credits are added as well) but you cannot take away clauses.
As Samson pointed out, it's generally 75 years after the death of the copyright holder - although for anonymous works, I believe it's 100 years after the date of publication. Either way, you're unlikely to be able to use the work within your own lifetime.
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