1) Is this offensive or defensive for the purpose of Foci?
Is the zone border there to stop people from moving (offensive) or to keep them from hurting you (defensive).
2) Does a caster need to add the zone wide cost to the spell, ie using part of the spell power for a zone wide effect? Example: 10 strength earth magic spell - 8 on the Block, 2 on making it Zone Wide?
For a simple border between two zones, no. If you want to put up a zone border around an entire zone, I'd say yes.
3) Could the caster then spend time creating a ward on this new zone border with Thaumaturgy? I understand time would be a problem but considering that he could turn around and cast another spell to increase the duration by 10 exchanges, this doesn't seem to be an issue?
A ward needs something more tangible to be anchored into than a zone border. Something more permanent, like a threshold. There might be places that have a 0 threshold, which means they are nothing much in terms of a barrier, but they do allow for a ward to be anchored on them. Usually that's houses, places where people live, but on occasion, there might also be natural places that can serve that purpose. But just throwing up a zone border wouldn't do the trick in my book.
4) Does this count as a violation of the laws? It doesn't seem to as the affect won't be killing or even causing harm directly?
All depends on what the ward does. If the ward shoots lazors at anyone who enters, and a mortal enters the ward, that's a violation of the first law right there. If it's just a detection web, or maybe a "you don't want to be here" suggestion type of ward, it won't break the laws.