It would be a nominal charge for storage, bandwidth, and indexing. Book stores carry public-domain titles, for profit, and most have no issue with that. You can always procure the same files somewhere else—they are public domain, after all. Those who pay are doing so for the convenience, not because they’re forced to.

They could stick to public domain & indie titles. They won’t, but they could.

When it comes to their trademarks Valve can’t take a fully hands-off approach without negative consequences. Either they explicitly endorse the use of the Portal name and other branding, in which case they’re encouraging and aiding the project and could potentially be caught up in any lawsuit from Nintendo, or they say nothing and allow the trademark to lapse from non-enforcement, or they prohibit the project from using the Portal branding and enforce that prohibition with a lawsuit if needed. Unfortunately for the project, only one of these options retains their trademark and doesn’t set them up for a fight with Nintendo.

Look up the legal principle of estoppel. In general you can’t turn around and sue someone for doing something after informing them (in writing no less) that you’re okay with it, even if you would otherwise have had a valid basis to sue.

So you’re not remapping the source ports to be unique? There’s no mechanism to avoid collisions when multiple clients use the same source port? Full Cone NAT implies that you have to remember the mapping (potentially indefinitely—if you ever reassign a given external IP:port combination to a different internal IP or port after it’s been used you’re not implementing Full Cone NAT), but not that the internal and external ports need to be identical. It would generally only be used when you have a large enough pool of external IP addresses available to assign a unique external IP:port for every internal IP:port. Which usually implies a unique external IP for each internal IP, as you can’t restrict the number of unique ports used by each client. This is why most routers only implement Symmetric NAT.

(If you do have sufficient external IPs the Linux kernel can do Full Cone NAT by translating only the IP addresses and not the ports, via SNAT/DNAT prefix mapping. The part it lacks, for very practical reasons, is support for attempting to create permanent unique mappings from a larger number of unconstrained internal IP:port combinations to a smaller number of external ones.)

What “increased risks as far as csam”? You’re not hosting any yourself, encrypted or otherwise. You have no access to any data being routed through your node, as it’s encrypted end-to-end and your node is not one of the endpoints. If someone did use I2P or Tor to access CSAM and your node was randomly selected as one of the intermediate onion routers there is no reason for you to have any greater liability for it than any of the ISPs who are also carrying the same traffic without being able to inspect the contents. (Which would be equally true for CSAM shared over HTTPS—I2P & Tor grant anonymity but any standard password-protected web server with TLS would obscure the content itself from prying eyes.)

No, that’s not how I2P works.

First, let’s start with the basics. An exit node is a node which interfaces between the encrypted network (I2P or Tor) and the regular Internet. A user attempting to access a regular Internet site over I2P or Tor would route their traffic through the encrypted network to an exit node, which then sends the request over the Internet without the I2P/Tor encryption. Responses follow the reverse path back to the user. Nodes which only establish encrypted connections to other I2P or Tor nodes, including ones used for internal (onion) routing, are not exit nodes.

Both I2P and Tor support the creation of services hosted directly through the encrypted network. In Tor these are referred to as onion services and are accessed through *.onion hostnames. In I2P these internal services (*.i2p or *.b32) are the only kind of service the protocol directly supports—though you can configure a specific I2P service linked to a HTTP/HTTPS proxy to handle non-I2P URLs in the client configuration. There are only a few such proxy services as this is not how I2P is primarily intended to be used.

Tor, by contrast, has built-in support for exit nodes. Routing traffic anonymously from Tor users to the Internet is the original model for the Tor network; onion services were added later. There is no need to choose an exit node in Tor—the system maintains a list and picks one automatically. Becoming a Tor exit node is a simple matter of enabling an option in the settings, whereas in I2P you would need to manually configure a proxy server, inform others about it, and have them adjust their proxy configuration to use it.

If you set up an I2P node and do not go out of your way to expose a HTTP/HTTPS proxy as an I2P service then no traffic from the I2P network can be routed to non-I2P destinations via your node. This is equivalent to running a Tor internal, non-exit node, possibly hosting one or more onion services.

It is not true that every node is an exit node in I2P. The I2P protocol does not officially have exit nodes—all I2P communication terminates at some node within the I2P network, encrypted end-to-end. It is possible to run a local proxy server and make it accessible to other users as an I2P service, creating an "exit node" of sorts, but this is something that must be set up deliberately; it's not the default or recommended configuration. Users would need to select a specific I2P proxy service (exit node) to forward non-I2P traffic through and configure their browser (or other network-based programs) to use it.