2) is handled by the security of the modem/router. As far as I know the guest networks in Airport routers have zero access to other networks and can easily be configured to disallow guests from even connecting to other guests. Whether there are exploits has yet to be seen, but if they are on different networks I doubt this would be easy (or even possible).
3) may be configurable depending on modem/router, the bandwidth available to the modem is often much greater than the bandwidth actually purchased by the customer (i.e., it isn't actually shared bandwidth).
4), 5), 6) are all being handled by some ISPs already. For instance, if you have Comcast you can already log into guest networks in other customers' modems that support it (it's opt-out and limited to modems that support wifi). Presumably 7) would be handled more/less in this way, but it is really up to the ISP's billing department to keep track of and is of little interest to us here.
There is no answer to 1) yet that I'm aware of (operating much like cell towers operate). But there would also have to be enough wifi points available to make it even practical to rely on. Additionally, 8) may be an issue with older modems/routers who can barely handle the network as it is. However, it would only be implemented with compatible modems so this issue would likely never be a problem (unless your shiny 2014 model modem/router is actually broken or something). And like 7), it is really more of an internal thing for the ISP to figure out.
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It would be trivial to have guest network set up on a separate IP address if it is built into the modem. The only potential issue is the limited number of IP v4 addresses, which would be less of an issue if more websites supported IP v6 (although tunneling would make this much less of an issue, but I don't know if you can tunnel v4 over v6my knowledge of this is more limited, so don't quote me on it).