A TP-Link FAX states, 

A guest network allows visitors in your home to use the internet without needing to connect to your private network. Wireless devices connected to a guest network have no access to resources shared on the main network so that the security and privacy of the main network are guaranteed.

Because of that and other curiosities, I have three probably very basic questions:

(1)  What would be the problem with a guest connecting with my private network, and what is an example (or are examples) of "resources shared on [my] main network" a guest can "access" that I would need to be concerned about?  I mean, it'd really be only one or two "guests," and they're both long-term and trusted friends.  -- and

(2)  Posting here provides the option that I identify my "Hardware Version" and "Firmware Version" (I'd suppose to identify one's equipment to facilitate better help).  My iPhone app states "Version 1.9.17."  Is that the Hardware or Firmware version, and how or where would I find the OTHER version?

(3)  My main module of course receives an Ethernet input from my ISP's modem.  I then have the other Ethernet port going to an 8-port switcher.  Would my other two modules accept only an input and (facilitate) an output, or can I use both ports on my other two modules as outputs to, say, Roku TV's, Laptops, garage door gateways, Philips Hug bridges, et al.?  So does each module have a dedicated in and out port, or can both be used as outputs to connect devices via Ethernet?  And yes, I guess I could simply plug something in both ports and see but if I fry something, I'd only have myself to blame that I didn't ask for guidance here first... haha

Thanks all. - Mike

[Just saw after posting this:  I can actually buy a single module from Walmart for $85, but my question above still stands.]