Can I use the IP address range used on the side of Arcstar Universal One site for the stack route of a VPN gateway? Can I make an advertisement forcefully even when the addresses are duplicated? I am assuming a case where I use the longest match to make communications to the ECL2.0 side.
You can make such settings. However, it is your responsibility to design the routing in the whole network when you duplicate addresses. Note that you cannot duplicate the addresses in the connection section between ECL2.0 and Arcstar Universal One (/29 that is specified by you).