It's not "trade" if you're going into occupied territory, setting up a base of operations, and acting in a manner that exclusively benefits your country.
That's at best an occupation. At worst, it's an invasion.
Now, if they were invited to set up a bar, that's another thing entirely. It's like vampires: they're only welcome if you invite them in.

See, that's why I specified "If the Emperor ever gets up." The Imperium doesn't work the way the Emperor would have it work.
The original 12 Inquisitors were chosen not by the Emperor, but by the Hero (whose name I can't remember, but his official title, as declared by the Emperor himself, is the Hero), who, for all of his fantastic work, was still a mortal man, and his choices for the twelve were also mortal men. Men are subject to failure, and while the 12 served admirably, the Imperium has decayed (morally and physically) in the millennia since the Horus Heresy, due to the lack of direct leadership from the Emperor.
The Ecclesiarchy and the Inquisition are institutions the Emperor would have been... unlikely to approve, but they now stand as the most dominant forces in the Imperial government, and it is due mainly to their influence that the policy of "Kill all Xenos" is in effect.
As for the potential issues with dealing with Necrons... there aren't as many Necron leaders as most fans of the faction would like. A few well-aimed shots from five miles away, and you take down their armies, because only the sentient Necron Lords are capable of issuing commands, and a Necron army without a commander is effectively a bunch of standing corpses.
And dealing with the 'Nids is a matter of resource management. If you can manage to convince the Tau that using an Exterminatus to wipe out a planet infested with them prevents more dangerous conflict from occurring (so that it serves the greater good by preventing even more senseless death in the long run), then all you have to do is evacuate as many as possible before an Exterminatus on any 'Nid infested planet. Wash, rinse, repeat, until they realize they can't get a decent food source in this galaxy, and they give up and look for more suitable hunting grounds. At their most complex, 'Nids still have the thought processes of animals. If they can't get food hunting somewhere, they'll look somewhere else.
Orkz, though, are impossible to wipe out permanently, unless you manage to convince enough Orkz that the Orkz can't possibly exist.

Now, for your ideas.
The 'Nids were from outside of the galaxy. They're a universe-wide problem. Unless the C'tan have far more power and influence than even the Daemon Princes have been shown to possess (which isn't fucking likely), 'Nids aren't the work of the C'tan. Unless they ate the C'tan, in which case the entire universe would already be dead.
Finally, to address your last bit of speculation: the Omnisiah (the one worshiped by the Tech Priests) has actually been co-opted by the Emperor, who the Tech Priests recognize as being the living embodiment of the Omnisiah.
There is, however, a C'tan at the core of Mars, the planet the Tech Priests had originally sequestered themselves to. I'm not sure this one's name has been revealed, but it's possible it's the Nightbringer.

And that info's coming from a guy who can't afford to actually get into the game. The fucking models are expensive. I've gotten a bunch of info from a variety of sources, including basically scouring the Warhammer 40K wiki, but I'll likely never actually put that info to any real use.
Still, the lore itself is a worthwhile read.
Reading a fair bit of the lore is why I'm convinced that if the Emperor ever gets fully restored, he'd forge an alliance with the Tau. The Tau have an extremely small warp presence; Daemons can't possess/influence/psychically detect them, and they have no ships capable of utilizing the Warp for rapid transportation, likely due to their lack of ability to manipulate Warp energies (meaning there are no Tau psychers). This makes them immune to the temptation of the Ruinous Powers. They're also fairly naive, so entering a treaty with them would be easy, especially with someone as good with people as the Emperor.
Add to it that the Tau are some of the best marksmen in the galaxy, with insanely powerful ranged weapons, decent armor, and the facilities to mass-produce both, and you'd end up with an alliance that could both spread the will of the Emperor, who only wants the greatest possible good for his subjects and allies (which is something the Tau would instantly respect, what with their philosophy literally being "Serve the Greater Good"), and potentially stop most major threats to the galaxy as a whole (possibly save for the 'Nids, because they are allegedly more numerous than the stars in the universe, and almost certainly save for the Orkz, because they are literally mushroom men).

See, that's why I specified "If the Emperor ever gets up." The Imperium doesn't work the way the Emperor would have it work.
The original 12 Inquisitors were chosen not by the Emperor, but by the Hero (whose name I can't remember, but his official title, as declared by the Emperor himself, is the Hero), who, for all of his fantastic work, was still a mortal man, and his choices for the twelve were also mortal men. Men are subject to failure, and while the 12 served admirably, the Imperium has decayed (morally and physically) in the millennia since the Horus Heresy, due to the lack of direct leadership from the Emperor.
The Ecclesiarchy and the Inquisition are institutions the Emperor would have been... unlikely to approve, but they now stand as the most dominant forces in the Imperial government, and it is due mainly to their influence that the policy of "Kill all Xenos" is in effect.
As for the potential issues with dealing with Necrons... there aren't as many Necron leaders as most fans of the faction would like. A few well-aimed shots from five miles away, and you take down their armies, because only the sentient Necron Lords are capable of issuing commands, and a Necron army without a commander is effectively a bunch of standing corpses.
And dealing with the 'Nids is a matter of resource management. If you can manage to convince the Tau that using an Exterminatus to wipe out a planet infested with them prevents more dangerous conflict from occurring (so that it serves the greater good by preventing even more senseless death in the long run), then all you have to do is evacuate as many as possible before an Exterminatus on any 'Nid infested planet. Wash, rinse, repeat, until they realize they can't get a decent food source in this galaxy, and they give up and look for more suitable hunting grounds. At their most complex, 'Nids still have the thought processes of animals. If they can't get food hunting somewhere, they'll look somewhere else.
Orkz, though, are impossible to wipe out permanently, unless you manage to convince enough Orkz that the Orkz can't possibly exist.

Now, for your ideas.
The 'Nids were from outside of the galaxy. They're a universe-wide problem. Unless the C'tan have far more power and influence than even the Daemon Princes have been shown to possess (which isn't fucking likely), 'Nids aren't the work of the C'tan. Unless they ate the C'tan, in which case the entire universe would already be dead.
Finally, to address your last bit of speculation: the Omnisiah (the one worshiped by the Tech Priests) has actually been co-opted by the Emperor, who the Tech Priests recognize as being the living embodiment of the Omnisiah.
There is, however, a C'tan at the core of Mars, the planet the Tech Priests had originally sequestered themselves to. I'm not sure this one's name has been revealed, but it's possible it's the Nightbringer.

They don't have enough resources to actually hold the Mojave.
They can't even get rid of the fucking Fiends without the Courier's intervention. If you don't wipe them out, the NCR ends up fighting a battle on three fronts instead of just two during the final fight, and this goes disastrously for Camp McCarran.
They mean well... but the road to hell is paved with good intentions, and not being able to secure your territory is practically a death sentence for everyone living there, especially in the post-apocalyptic wasteland that Fallout is set in.

I've seen people give him welding goggles. And he wears them.
Either the "only dog armor" thing is bugged, or your character can also wear dog armor, despite not being a dog.

Modding adds or alters information already present in the game. It's modifying the game as it exists.
Extracting data from the game for alternate uses, i.e., ripping music to have the music, especially when there's an option to purchase the music outright, isn't modding.