I believe the correct Port Forward mapping is to forward the SIP port used by each different extension to the particular computer/device that extension is on. I.E. if Axon on PC1 is using 5060, forward 5060 to PC1.. If Extension 102 with port 5070 is on PC 2, forward 5070 to PC2. Keep in mind though that I don't think WAN to LAN port forwarding will help much for internal calls, I think it's more to help calls to/from your VoIP provider.
Note: Failure to connect to the STUN servers is often indicative of firewall port blocking / incorrect port forward set-up. You can also use UPNP as alternative. UPNP is not available on all routers, and even on the routers that support it, the functionality is sometimes limited. Having said that, UPNP is a great feature in theory that helps take all the hassle out of port forwarding and network set-ups. I don't know exactly how it works but my understanding is it allows free communication through the firewall on specific ports used by the software (auto port forwarding?).
Also make sure that your internal network (LAN) is not blocking anything (or at least not any ports related to Express Talk).. if your network is for home or a small business, there really shouldn't be a need for a LAN to LAN firewall as most if not all the communication between PCs in your network would be trusted. In any case, this sounds like it could be one of the major problem(s) you're having as the difficulties you are experiencing are all internal.
Lastly, it is very unusual for Express Talk to report "error returned: 401 unauthorised" and then register fine later using the same username/password. I have yet to come up with a plausible explanation to this, it's almost as if line interference is causing corruption to the SIP messages so they have to be resent several times before they get through (would also explain the delay?). Perhaps blocked ports can cause similar issues.
Since I haven't come across these problems before, it's hard for me to identify exactly what the source of the problem is. But it seems to me the most plausible explanation is either blocked ports on the internal LAN and/or packet corruption possibly due to interference and/or network issues. Line interference doesn't sound right though since you mentioned calling 102 from 101 doesn't cause any problems.