I hope it's not too late. For the first question - you should be able to ping the router. Then, you will pass the traffic the same you pass the traffic now, towards your main router. But, if you want only those four machines to route the data there, just create rules in respective firewall. If you have to kill the existing tunnel, you have to think of another way - new VLAN or new tunnel. I didn't understand, however, the big picture - why MPLS in the middle of the normal LAN ? You need to use MPLS paths and respective protocols to manage the traffic from the MPLS router.