Can I use Mirror Strike on Ink-treader Nephilim when it's not attacking?
Yup, unfortunately you're right. Unblocked means attacking and unblocked, after blockers are declared.
This is in the comprehensive rules for the declare blockers step:

509.1h An attacking creature with one or more creatures declared as blockers for it becomes a blocked creature; one with no creatures declared as blockers for it becomes an unblocked creature. This remains unchanged until the creature is removed from combat, an effect says that it becomes blocked or unblocked, or the combat phase ends, whichever comes first.

And there's a friendly summary from the glossary at the end:

Unblocked Creature
An attacking creature once no creature has been declared as a blocker for it, unless an effect has caused it to become blocked. It remains an unblocked creature until it’s removed from combat or the combat phase ends, whichever comes first.

So your opponent could make good use of Mirror Strike, letting your whole team hit in, targeting the Nephilim (and thus the rest of your team), and sending it all back at you, but... I don't think that's quite what you were looking for.