I'm a bit dumbfounded on the "why not go to DXB" thing.
1) Great circle between CDG and SIN goes nowhere near DXB
2) Emirates A380s are Engine Alliance powered; SQs are Rolls powered. Ergo there isn't a spare engine in DXB.
Why go back? Well I guess because there are not many diversion airfields in Siberia that can house 444 pax should the aircraft have to divert for a second engine problem or, indeed, any other issue.
3-engined ferrying as you know, joetom, is a different ballgame from carrying revenue pax.
Also there may be limitations on which diversion airfields the A380 could get into but I'd be very surprised if, as suggested by a previous poster, this was a runway strength problem. After all, the A380 has a lower ACN than a 77ER, does it not? More weight, yes, but more wheels, too!