I don't know about rejecting a delegation... I haven't scanned the ai code. But if your sending a delegation increases your intel, shouldn't their delegation to you do the same? If this game were more polished, I would say that, yes, absolutely, your acceptance of their delegation affects their behavior aside and apart from any behavior adjusted due to diplomacy modifiers. But, this game has so many burps in it, just absolutely absurd lazy-hack mistakes, that its anyone's guess as to whether or not that what should be, is. Since its cast in shadow, I would suggest *acting* as though your acceptance of their delegation will make their decision regarding you more informed. In other words, if you are playing it safe, don't accept.
Same holds true for revealing your capital. I don't know how much of the map the ai is allowed to consider at any time during the game... I don't know if your capital is actually 'invisible' to them. But I *do* know that the ai scouts beeline for city-states. I've followed them. At the beginning of the game, I think the ai picks a strat, knows where all of the city-states are, knows which ones will give free envoys, then assigns value based on the the victory condition they are going for, makes a list, and then tries to run down that list with its scouts/warriors. But does this mean the ai takes your start position into account if you haven't shared your capital's location? Again, if I'm playing it safe, I don't give out that information.
In my current Brazil ynamep giant TSL, couple of 100 turns in, I had 2 land units (starting warrior and scout), and refused to give out my capital's location to Norway. Just in case. Cause you never know. On the other hand, I had prepared for this, and when I made 2 galley's while power-rushing disembarkment, since I had the 100% card in, I made 2 more galleys (6 turns a pop) for the exact purpose of making Norway happy... because one time when I spent 100 turns getting to England, Norway dow'd on me the turn after meeting him and killed my unit. So along with my 4 galleys, I made some good trades with him, and gifted in open borders and 10 gold. Which means I got to Stockholm, for x2 great people points per district (which has netted me all of the great merchants so far - I also power-rushed commercial districts, and so far I'm the only one that has them). But I DID NOT tell him where my capital is.
Cause you never know.