I'll try and answer both questions but you have removed the diagram so I'll have to do the first question from memory :)
If the service switch connects at L3 to the core switches then, assuming you setup the routing between DS2 and the service switch as discussed, DS2 would receive routes for the voice gateways from both the core switches and the service switch.
Assuming the interconnects from DS2 are the same speed to the cores as to the service switch then DS2 should favour the routes received direct from the service switch which is what you want. In fact you could then use the cores as a backup in case the trunk failed if that is what you wanted.
If the interconnects are different speeds then the above may not apply.
If you do not create this new vlan for peering the traffic will not traverse the link because the traffic is not arriving at DS2 as L2 traffic unlike the guest vlan traffic which is. The traffic from vlan 50 is L3 traffic when it arrives at DS2.
It might help to think of it from DS2's perspective. A packet arrives from vlan 50 via the routed link. DS2 does what any L3 device does and does a route lookup on the destination IP but you don't have those routes if you don't peer with the service switch (assuming you are not still receiving those routes from the core switches).
So DS2 cannot forward the packets. The native vlan does not come into it here.
Jon
