Any question answered...

Could I stick a washer / spacer on the hub body before putting the cassette on? That would push it over enough...

Not a good plan because a: the lock ring threads will have less engagement, and they are hardly over engaged to start with and b: the chain will probably hit the dropout when you engage top gear.

So it's indexing correctly from 1-9, but won't go into 10?

(Add 1 for 11 speed).