Could you explain these statements a little more?
Without running the system long enough to cause undercutting you can't really tell without digging.
What is undercutting? I assume you are referring to the slab here?
Is it best to try to find ANY piping at all leading under the poured slab and reroute it? You are right, that is a lot of shovel work, but certainly necessary IMO to avoid another head leaking everytime the sprinkler comes on.
How would I know if the 3rd line exposed is THE main line? Is there a test? How do I know if another main line exists? Is there a method for searching (like finding the boxes before)?
Or your line may have been put in as a loop to help with pressure and flow.
What are implications of a loop? How can I know if a loop was put in? If a loop was put in, what does this mean in rerouting the 3 lines found thus far? Will it do nothing?
I can't say thank you enough for your help thus far!