wrong a lvl 1spy could pass lvl 40 borders but higher your borders are to there spy lvl smaller the chance spy can get in. Ans to question yes borders help catch spies so can let you know of possible attackers. Also if you goto espionage in comand centre and click on counter-espionage you can see spy's that are watching you and remove them.
Also whatever your borders you can see an army thats attacking you once it's an hour or less away from reaching you. But depending on spy lvl to your borders you cannot see structure of army. e'g =lvl-borders you will not see structure of army till 40 mins away for every lvl spy higher is another 10 minutes. So is spy is 2lvls higher you will not see structure till 20mins away. At 10 mins away you should be able to see structure whatever the difference is between borders and spy.