By "alien" I really did just mean "different". There are comprehensible possible minds that are nothing like ours.

If we imagine the designer is not acting irrationaly or random - then all potential motives go into survival/reproduction and max. p/p.

I don't think this is true. Imagine Omega comes to you and says, "Look, I can cure death - nobody will ever die ever again, and the only price you have to pay for this is a) you can never have children, and b) your memory will be wiped, and you will be continuously misled, so that you still think people are dying. To you, the world won't look any different. Will you take this deal?" I don't think it would be acting randomly or irrationally to take that deal - big, big gain for relatively little cost, even though your (personal) survival and reproduction and (personal) max. p/p. aren't affected by it. Humans have complicated values - there are lots of things that motivate us. There's no reason to assume that the simulation-makers would be simpler.