No, there's no correlation between the number of NPCs and any presence of artifacts Default maps assign artifacts to 15% of the planets (users can change this in custom made maps). Likewise, NPC defenders around planets are randomly picked from some existing templates

The AI doesn't explore everything right away, but they do get around to it eventually. You'll know when they find artifacts, as the planet will have a tringle symbol underneath and you'll get an event message. Captured homeworlds are never worth exploring because there's never anything on them, but even non-artifact planets can have other bonuses that you won't know about if you don't explore.

It varies, can be either. I've looked in the files for someone else who asked the question last week, and I didn't see anything that specifies it.

So, my educated guess it's a 50/50, you either get it on the first or second (I've had both cases plenty of times)

I love the artifact that gives you a 25% discount on all research, thats totally worth the money

I dunno, Arseface420 (does that mean you get "sh*t face baked?"), I mean you are right that you want a lot of resources, but that cash can come in really handy. For example, instantly leveling up your new capital ships before they even reach the battle, putting bounties on your enemies (especially the ones with imminent pirate attacks that you are fighting anyway), and buying the crap you need on the black market are all handy. Obviously you neeed to be well balanced, but I generally find a use for all those excess credits. I espcially like that you can upgrade your cap ship experience with cash alone.

The way I play TEC I don't buy any seige frigates because they go down too quick and just buy a bunch of Dreadnaughts to take out enemy colonies. They always have some good abilities by the time they get there... Then you have a good way to get the exact planets you need (volcanic or ice) and they have already been cleared for you. Its a decent mid-game strategy on single-player anyway. Plus, pirates always go for trade ports so having a bunch of logistics structures like that gives you more time to get your fleet to an important terran planet before all your good structures are nuked.