Use either switches or death counters.
For the end of one bound obstacle and the beginning of another, you have that "rest" area correct? You should have a location for that "rest" location in order to detect that the player has beaten a certain obstacle in order to continue on. Make another trigger that at the end of each obstacle, center the location 'spawn' on a non-existant unit at "rest" area. This will cause the location 'spawn' to move to the center of the "rest" area so you will need one spawning trigger.
The spawning trigger would be like this:
| Trigger |
| Conditions: |
¤ Command 0 Zergling.
|
| ¤ Accumulates at least 1 mineral. |
| Actions: |
¤ Create 1 Zergling at 'spawn'
|
¤ Subtract 1 mineral.
|
| ¤ Preserve trigger. |
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Bah legacy rapes me with his triggering -.- stop being so fancy and saving locations -.-