YouTube how to for SmartRG SR350N as I guessed as is the case with a lot of routers about the default log-in details. Or SmartRG SR350N Access Control Passwords Router Screenshot - PortForward.com
You most likely do not want to fiddle with the security settings, however this should at least show you how to log in. (If there is any port forward or UPnP option then it will probably be within the Advanced menu section.)
According to this Router Screenshots for the SmartRG SR350N - PortForward.com there is a UPnP section. Check it to be sure it is enabled. If it's not then that is probably at least part of your problem solved. If it was already enabled, then (there does not appear to be port forwarding option from what I can see) see the port triggering section which is comparable to port forwarding but only opens a port(s) when the application opens (WireShare will use one port for Gnutella and a different port or range of ports for Torrents.) If you use 'Port Triggering' you will also need to change WireShare's settings for listening port from UPnP to port forward and add the equivalent port number(s).

Keep in mind Windows XP has its own software firewall. You should check your Control Panel -> Firewall to see if WireShare has an exception rule. If not, use the Add program exception option and find the WireShare program in the option it gives you.

I would tend to confir with ukbob's response about possible ISP bandwidth shaping aimed at p2p file-sharing. Looks like your torrents are also affected for speed, so if the ISP is targetting file-sharing, they might be targetting multiple file-sharing protocols.
But also check your upload settings in WireShare.

It is possible to transfer files on gnutella network without being connected. Connection is definitely required for searching. Over the years I've seen uploads & downloads occur whilst disconnected. Torrent transfers can occur because torrents use a different file-sharing protocol for connecting. Connection to torrent trackers (and seeders) is all that's required for torrents.

Note: if you also installed the Hostiles with WireShare (this is recommended), it would be a good idea to switch to standard 'Light', not the total Japanese block.
You can force WireShare to use a fresh connection file simply by deleting the gnutella.net file from the Application Data\WireShare folder but WireShare must be closed whilst you do this. You can easily access it via using the "Run..." tool from Start menu & copy-paste in
%appdata%\WireShare

As far as hosts co-sharing a particular file, WireShare does not necessarily list all the hosts. This is something rather strange in the LimeWire-5 re-coding of how the search results are displayed.

I'd be curious if you could connect using Phex. (Note: Phex does not have UPnP support.)