TCP/IP – Understanding End to End Protocol

It may help if you think of the port as an extension of the IP address. Just as two different buildings may have a room 101, the rooms are not the same room.

10.1.2.3:1234 is a different port than 172.16.17.18:1234. The function of the port may be the same across IP addresses or protocols (e.g., IPv6 ports have the same functions as IPv4 ports, even though they are different protocols).

AS for point 1. well, maybe you could craft a IP header which check sum result would be 1, looking at the algorithm used (which is described in the RFC you linked), but if you expect somebody to do it for you, good luck...

Regarding point 2. you have to look at RFC2119 where it is said :

5. MAY This word, or the adjective "OPTIONAL", mean that an item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item. An implementation which does not include a particular option MUST be prepared to interoperate with another implementation which does include the option, though perhaps with reduced functionality. In the same vein an implementation which does include a particular option MUST be prepared to interoperate with another implementation which does not include the option (except, of course, for the feature the option provides.)

So it is up to the person that implement its version of an IP stack to decide which errors are reported or not. I have no idea if someone ever wrote an IPv4 stack with checksum error reporting trough ICMP but I doubt it.

My understanding is that if a checksum is bad, the packet should be dropped, and that's all. Re-transmission will be managed by upper layer if needed (generally by TCP).