Yes you can share the IP address among multiple PUs.
The easy answer to (1.) is that it's the only way to provide more than 255 LU Sessions (the limit for a PU Type 2 device) at a single IP address. For (2.) the answer is that it hints at the more complete answer below. And the answer to (3.) is that you can't, that it shouldn't matter, only what LU it connects to, and again see more below.
In the simplest design, all of the LUs would be dynamically created, so would effectively be one large pool, and you wouldn't care which IP address or token adapter the client attached to. For most people, some or most of their TN3270 works this way. But most also have some applications for which they need to have a deterministic mapping between TN3270 Client and SNA LU. So they will have multiple IP address's, some representing pools of LUs, and some representing specific, or small groups of, LUs.
Within these constraints there are options for how the specific LUs are created. Ranging from static pre definitions, through the lu-seed wildcard naming shown in your sample, to taking the names directly from TN3270E Clients. The last of these allowing the possibility of moving all sessions into the large pool model, but at the cost of implementing and maintaining client configurations. There's more on this, and better addressed, in the design guide.
What I think is important though, is that it is the IP side of the TN3270 Server that can provide the better hot standby - high availability solution. Because it's a TCP connection, any of a variety of load balancing products or features, from NAT, Local Director, Distributed Director,... through CSM and CSS can be used to send connections to the best and/or an available TN3270 Server. As a starting point you may want to consider the advantages of all clients connecting to DNS supplied names, rather than to a coded IP address.
