1. If you want the solution of IPv4 packets being a subset of the address space of IPv6, your options are rather limited in regards to tampering with the original packet. IPv4 hosts can't deal with the larger addresses of any IPv6 hosts they're talking to.
2. Of course there's no immediate benefit for IPv4 clients - there isn't in your proposal either! It's a compatibility measure, allowing IPv6-only clients to talk to IPv4 servers. (I make that distinction because under no scheme can an IPv4 host initiate a connection to a host with no IPv4 address.)
3. I have no idea what you're talking about with the idea that NAT64 requires IPv4 clients to talk to each other over an IPv6 subsegment. I suspect you're thinking of a different transition technology solving a different problem. NAT64 provides a virtual IPv6 subnet containing the entire IPv4 address space. IPv6 clients can send packets to this subnet, at which point they are routed to the nearest NATting gateway and passed into the v4 internet after packet rewriting.
4. FTP and SIP containing IP literals is an irreducible incompatibility, which cannot accommodate an address size change without intrusive packet rewriting anyway.
2. Of course there's no immediate benefit for IPv4 clients - there isn't in your proposal either! It's a compatibility measure, allowing IPv6-only clients to talk to IPv4 servers. (I make that distinction because under no scheme can an IPv4 host initiate a connection to a host with no IPv4 address.)
3. I have no idea what you're talking about with the idea that NAT64 requires IPv4 clients to talk to each other over an IPv6 subsegment. I suspect you're thinking of a different transition technology solving a different problem. NAT64 provides a virtual IPv6 subnet containing the entire IPv4 address space. IPv6 clients can send packets to this subnet, at which point they are routed to the nearest NATting gateway and passed into the v4 internet after packet rewriting.
4. FTP and SIP containing IP literals is an irreducible incompatibility, which cannot accommodate an address size change without intrusive packet rewriting anyway.