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https://documentation.meraki.com/?title=MX/Other_Topics/IPv6_Support_on_MX_Security_%26_SD-WAN_Platforms_%255BCore_Fundamentals%255D#OverviewUnfortunately, the IPv6 for High-availability (HA) is not supported at this time.
Perhaps not, both port forwarding and 1:1NAT won't change source IP;I think you may consider using Client VPN to assign the client with internal private IPs to enable them to access your internal server anywhere.