well, on the empire thing, im not sure really what the point of an empire is (other than exchanging strats or havin ppl to talk to...).
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Empireing is just another level of metaverse competition. You're in it with your mates instead of alone. Plus it's people to talk too, exchanging strats. Some like to write funny and good stories. Etc.
I've never foreign powers surrender either. I've tried being the most friendly with em, ripping em up militarily, etc. What I usually do to mop up is get into war, then demand planets for peace.
I used to get caught flat footed like you all the time. I started building the corvettes and starfighters before I wanted to. It probably costs 1 or 2 planets or 3 even. But on Gigantic that's not so big a deal. That solved it for me. I still get some demands but usually from civs across the map that can't reach me. By the time they can they are more than ready for peace. hehe. And do send out frieghters as fast as you can. They help too.
Also what might also help you is going for corvettes early, then exchange the tech for tons of other techs.
I usually go right to trade in the yellow, then right to the tech that gets you aphrodesiac, built it and trade it all around, then go right to corvettes and trade them all around. It gets you some bux. It works on everything except tiny maso where you just get hemmed in. There I go for the vettes right after I get to trade in the yellow and forget getting any early wonders. Got to or you'll be at war fast.