Monster Hunter Freedom Unite (MHFU)

[Zed_Acid]

Sorry, I didn't want to extend the already long post, but:

Yes, we did.
In fact, we did everything from the guide and more we found on the net, this is included under
"All settings according to forums' solutions"

This means that we did everything we could find, even the outdated stuff, which obviously didn't work.

We also even synched our GFX settings and everything else, really, just to make sure we all have the exact same settings in PPSSPP.

Edit: Answered too quickly, you updated your post

To clarify: I can't port forward (And I tried every trick and guide, Tunngle doesn't help there either). This is not an issue however, since Tunngle emulates LAN, and outgoing / incoming I obviously do have IPv4, it's just a quirk of my crappy router and ISP that I am not allowed to port forward, allegedly because of the internal IPv6.

Also, yes, my LAN IP is IPv4 too, and I can play LAN games with my wife fine without Tunngle. Or with, doesn't matter, it works.
Tunngle obviously also assigns an IPv4 to everyone, and that, again, works just fine. Just not with all three.

Tunngle has the YELLOW smiley face for all of us, which according to the guide and everyone else is sufficient and has worked in the past for each and every game I wasn't able to host / play because of Port Forwarding without Tunngle.

I was afraid that this may be the issue, but that doesn't explain why only I can host or why me and wife / me and friend can play together just fine.

Another important fact I might need to mention: It's only my wife who breaks off after loading the quest, the friend and I stay together in the quest.

I'm pretty certain I'm simply missing some small but important thing here, probably on my wife's end (since she's the ONLY one who breaks off when we try 3 players) - though for the life of me I can't figure it out.
Firewall settings are good, but even when we tried switching them off completely on all three ends, no joy.