Question 1:
The resource system is within the parcels you own ,and near bye it to a certain degree. The theory of buying more parcels or upgrading could show more resources could be true ,but I haven't seen it happen personally with anyone I talk to. I would increase settlement size from hamlet to village for the population ,and the extra add ons such as the tavern etc rather then praying for a set of resources. Be warned the map is kind of wonky at the moment so dont get your heart set on a place until they fix the profession list ,and various other issues.
Question 2:
You do not select which direction the added parcels go. It distributes evenly around the settlement like a circle.
Question 3:
No you cannot see any hidden resources on the map the only thing your privy to see is what the town owns ,and has directly nearby it. I would not trust upgrading an area praying for things such as "Minerals" or "Herbs" However there could be the potential if you live near a lake you may see the "Water" tags show up the closer you get to it. However I haven't seen anyone's actually "change".
Question 4:
There is no specific mechanic as far as I know showing the direct part where county lines end ,and the others emerge. I imagine it will be a fun job for map makers in the future ,and towns with landmarks will have to be used to determine if your in the wrong part of Elyria so to speak. Towns will be a saving grace ,and a good start to orient yourself on the map. Another thing I forgot to mention is land tables will be a good mark on where you can build,buy etc. Making it a little easier to navigate ,and know which map areas are available.
Be warned in the Hamlet A from County A situation if Hamlet A ends up taking resources from County B ,and County B is aware of it there may be some issues.
The last answer to the distance issue is that if your nearby a border of someone else it is probably a decent distance away. Remember an inch in the map is a far distance ,and so is simple centimeters.If you found a settlement directly on the line for bordering someone else which there are some.... your probably fairly close to the other county ,and that could be a point of contention between you ,and that count in terms of resources or the start of a good relationship with trade. I Imagine some really small shifting with the towns. Not everything is going to be laser perfect.
A warning for growth on the border any parcels across a county or duchy border, will never be part of your settlement unless dictated by both counts or dukes in a sedecim.
Also sustainability is a meme ,and is broken. Dont worry about it much at the moment.