This topology is possible, but it is not a good practice. A VLAN is a separation for a broadcast domain (L2 only). You can place as many L3 networks in the same L2 LAN as you want but you are effectively sharing the broadcast domain with more hosts than you might want. Additionally, if you have a malicious user who is one subnet, they can create a broadcast storm that would wipe out all the other nodes in the L2 network.
Q-in-Q is not possible because end user of ISP connect with SW1 and port is access. End user usually donot use switch where vlan tag is remove . end user directly connect with sp's switch.
please explain me little bit more.