If say a runner has no icebreakers and makes a run on a sever that has ICE on it unless that ice "ends the run" the runner may pace it without breaking subrutines. (this is the way i understand it) And with Upgrades you may access 1 form the sever and the upgrade or just 1 or other. sorry if they are very noobie qustions  

Yes if the runner can't interact will the ice the subroutines unless he has the proper breaker and they will happen but if there was no end the run he will get past that ice.

When you access a server you get to access whatever you would normally be able to access and all upgrades. So if two cards are in a remote server you will get to access both.

This is why you should be careful in picking ICE for your Corp deck- you could have powerful or deadly ICE, but unless they kill the Runner outright, they're simply not going to stop him if they can't end runs.

"barriers"  are essentials to block the runners run or almost to make him/her to spend money breacking subrutines (if he/she has the apropiate icebraker) almost you like to make an acces to a trap asset… ;-)