Getting more support around each network is an interesting idea. But ...

I have made comments regarding Shareaza elsewhere so felt no need to respond at the time. Particularly since the Shareaza developers themselves don't appear to care in the slightest for the G1 network.

Depends on user I guess. But I rarely download from raza hosts.

Shareaza uses extremely old and inefficient code for the G1 network. Something the devs have refused to update for 15+ years (one or more Shareaza clones even totally dropped G1 support.) Shareaza causes considerable traffic and bandwidth overheads across G1. As an example, Shareaza uses uncompressed search messages. (To get an idea of how much extra that means, think of how much a plain text file can be compressed when zipped. We're talking 3-5 times amount of data with peaks even much higher. And that's only the data. Raza on G1 uses TCP and not UDP for most of its messaging which means it is slower and more traffic just to send a single byte.) I cannot call myself a dev so I cannot be more specfic about the details.

Shareaza's G1 ultrapeer technology is outdated and inefficient. If Shareaza's G1 implementation was updated considerably then the idea of running as ultrapeers on G1 would not come across as being problematic.

Ideally, the devs would somehow link certain G1 functions with their G2 code as there is no doubt not only considerable similarities but also some functions might operate in an identical fashion. Possibly using flags for which aspects of the code apply to each network.

I cannot comment regarding eDonkey.

In the early days, some involved in the open source LimeWire project were opposed to multi-network apps. Shareaza often used the majority of its bandwidth for other networks including torrents. The bandwidth for Shareaza uploads to G1 was often as low as 0.1 KB/s (or less.) How frustrating would that be to anyone.

However, GTK-Gnutella made the move to adopt G2 in leaf mode. As for further adoptions, I guess it depends on the devs involved. That also applies to the WireShare project. Perhaps a bandwidth test could be done to check if the specific host has enough speed and pipe width to handle and thus join more networks. Some apps tend to do speed checks of their own in any case but this will be dependent upon the other hosts abilities as well and is not an especially accurate method.

Keep in mind, not everyone in the world uses high-end fibre or similar. Due to their location some people are stuck with anything from dial up, adsl, dsl, satellite or wireless where the ability to handle high traffic levels and higher speeds is lacking.

Carrier level firewalling is an issue. Perhaps you could negotiate with them? Some VPN services also have this issue.
For the VPN services that don't, your router may well have a special section for handling VPN port forwarding. I would recommend this if using a VPN service (if UPnP cannot handle it otherwise.)