Hi Ed,
Wiring it all up is just a quick little test to rule out any unpredictable nasties WiFi may cause. It never hurts. 
As for a): have you tried installing Raspbian + Roon Bridge manually? (Little guide if necessary). This may (or may not) rule out any weirdness in the Hifiberry image (I2S DACs are not expected to have any latency).
And for b): I very much doubt whether the DAC+ should need a delay, but a future Roon release will have timing/delay setting for endpoints that cannot/will not convey this information to Roon core themselves.