Players are put on the map fairly randomly, but it isn't really big: there are only 145 provinces, for 8 players (I think), and it is a warp-around map, so you are likely to start with someone very closeby. The maximum distance between your capital and someone's else should be under ten provinces; eight is the maximum I think.
The witch thing was a random event: those happen randomly (duh), though the game can affect them - a nation with a Death scale is more likely to get plagues for example. So you didn't actually do anything, but the dice gave you this event (a good one too, unless the curse happens to target your important commanders).
Ah, diplomacy. There is no hard rule here, but you will usually agree to be neutral with your fellow players... except if you want to attack them early on, of course. Expansion tends to focus on beating the independents as soon as possible, but you might consider province trades after that. Doing too well could result in an early war, but it seems unlikely: having more provinces on turn 15 does not mean much, and if you do have more provinces, you should have the upper hand in a war.
A nation like Ermor (the death themes at least) has also trouble with its Dominion, since nobody wants to see their population dying off (and Ermor will likely have very poor scales). It can be worked around however, especially if the Ermorian player is willing to keep Dominion under control. If this player starts casting Utterdark and spreads Dominion like mad, it will be another matter altogether.