You're always responsible for what you, yourself and your computer does. There is a chance EFF/some other organization could help you out in case you end up in court, but that's a maybe, not a guarantee.

Harder to make this argument with encrypted distributed filesystems. If I'm storing a single chunk of an encrypted blob on Filecoin, am I responsible for the entire file even if I don't know what's in it, and I'm only storing a single fragment?

This depends on the jurisdiction you're in. I.e. Europe's GDPR argues that you need consent to keep someone's personal data. Encryption doesn't equate anonymization, so there's a potential liabity.

This.

Until a clear, precise answer to this question is available, it is unreasonable to expect individuals to take risks and seed.

It is one thing if an organization like IA gets in trouble with the law. They have money, lawyers, name recognition and are big enough to at least fight a lawsuit, even if they lose. Who is going to help an individual if he/she gets in trouble with the law, unknowingly? Am I expected to read through tons of complex copyright law and interpret it, just so I can seed a handful of items? No thanks.