Windows – How to send files via lan between two computers

No, it won't work for this configuration - the homegroup cannot span more than one network segment, and you have 2 segments here.

Try browsing on your upstairs desktop to the IP address of the downstairs machine prefixed with \\ in the start menu's search box. Say the downstairs PC has IP address 192.168.0.53 you would use:

\\192.168.0.53

And the resultant window should contain the shares.

You won't be able to go the other way as your laptop is in the way.

The first basic thing you can do is bring up a commmand window as well as browsers for your individual router admin page. When say you know the IP address of PC1 by typing ipconfig into the CMD window you can see your IP address, and needless to say this needs to be on the same network range as all your other devices (routers and PC2) to see them unless you know how to setup that connection on the routers to "Route" from one network range to another.

This is for if the router2 is connected to router1 through the LAN ports on both machines.

most of your internal devices I would say are on the 192.168.1.0 range.
As a possible solution have the Router connected to the internet setup for DHCP with an IP address of 192.168.1.1 and a subnet of 255.255.255.0 . Check PC1 is connected with that address as the default route in the CMD window with ipconfig. Next in the admin page for router2 make sure it has the default route setup as 192.168.1.1 with its IP as 192.168.1.2. Lastly in PC2 you can try and type Ping to all three of those IP addressses to see if it completes successfully.

After all that is done and you still have issues it may be because you need to type a double \ at the beginning of the url PC2\MySharedFolder

I would like to clarify my solution is to put everything on the same network IP/subnet mask range, because I believe that is the only way Windows workgroup sharing will actuall work; but I may be wrong on that.