Microstation V8i SS3 [OR] template troubleshooting: Can I go through the conditionals that make or break components?

Is your second corridor in separate DGN file, that does not have an active terrain set? This would mean that the side slopes have nothing to search for, and therefore do not get created.

Right off the bat, I had attached that terrain file but turned it off as it had been in my way, and ultimately I failed to set it as active terrain. After correcting this, the problem persisted.

In an effort to troubleshoot further on my own, I decided to tinker with some of the end conditions. One of them had "End Condition is Infinite" selected. I turned that off and reran it and the template drops were mostly fine.

Now I'm interested more in all the properties of end conditions and how they should be expected to work. In what scenario can "End Condition is Infinite" be best used?

The 'End Condition is Infinite' is the fall-back situation. For example, say you have 3 options when in fill, a 1:1 slope that tests for a height of 1m, then a 1:2 slope that checks for a height of 3m - these are fixed as they have a slope and a width/height. What happens when the road is over 3m high - this is where your final option (say) a 1:4 slope that is allowed to search 'to infinity (and beyond!)' to find the ground.
I would say that in nearly all cases you would have ONE 'End Condition is Infinite' case, for when the other options do not find the ground.

What about all the other options? How do they work and when do I use them or not use them? I was hoping to get more answers than questions. I gather from the lack of documentation mentioned that there is none. There should be better treatment of caveats.

While not as robust as I'd have liked, when I finally found the documentation for criteria file syntax, I became much more adept at writing those.

Another question I have (which I expect documentation should cover) is: why doesn't my template identify my plan lines that I've attached as "External Attachments"?

In the Template Editor, there is a Test button that opens a special window that shows the template on a dummy section. There is a target list that will be populated with any specific surfaces you may be targeting, and an Active Surface line. You can click on it and as you move the cursor into the section view, it shows what the end conditions would do when they find the surface. There is a slope option so you can see how a sloping surface would effect the results. If you are using display rules, you can toggle them between true and false to see those effects. And if you use some type of null point that targets a feature, you can select it and move it around to different places and see its effect.
Also, when in the editor, you can right click on a point and temporarily move a point horizontally, vertically or both to also test display rules. Before that tool, we used to delete constraints and certain points, use the move point tool to see the effects and then undo the deletion of the restraints.