1 (and all numbers except 9) is already there, so it can't be used.

All of the ?s in an expression will represent the same digit (0-9), and it won't be one of the other given digits in the expression.

js

what language?

test : "if more than one digit works, give the lowest one".
also test : "Testing for: ?35805+5466=?41271. Expected: 9, instead got: 1"
are instructions still correct?)

No, you're wrong, the tests are ok. Read the instructions again.