Yes. It supersedes what was suppose to happen. So it's an either/or. You either hit the 30 sec time limit on the rep before you hit the predetermined finish line OR you hit the lap button and it moves you to the rest interval. You can always hit the lap button early in an interval and it'll move you to the next interval in sequence.
Same for hitting the lap button on a normal run. When I do my Jack/Jill route, its something like 17 total miles up and down the same hill that's about a mile in length. But from my house to the top of the hill is 0.5 miles. So it would be annoying to try and keep track of the uphill vs downhill splits when they'd always be split down the middle. So I hit the lap button at 0.5 miles into my run. It's just tracking 1 mile intervals like normal, so the next update on my splits will come at 1.5 miles not 1.0 miles. This way I can track the pace variance of each of the ups separate from the downs.
Yes for the most part. When I write plans for others, I'll either use Daniels R value, or just mile pace. They tend to be close for most but not all. My R pace was 5:43 or 39s 200s. When I actually carried out the plan I was doing 150m splits which were around 30s, 300m splits around 60s, 450m splits around 90s, and 600m splits around 120s. So I was scheduled for 5:43 pace, but I was being consistent with around 5:14 pace. Since I was consistent, then I kept working in that area. I did do two workouts with Daniels Interval reps beyond 1000m in the 5:45 range as well.
But when I write training plans I don't tend to write them in distance per se. Rather I calculate what that person's R pace distance would be for 30, 60, 90, and 120 seconds. Then I tell Person A to do (100, 100, 200) and Person B to do (200, 200, 400) but if Person A and B compared their plans they may find that their workouts are the same (60, 60, 120) just as an example. Somewhere in the 3rd edition book Daniels suggests not going beyond 2 min of R pace.
Around 2-3 min at the soonest. So whatever distance you cover in 2-3 min is the distance that I'd start to trust the GPS as more reliable than <2 min. There's always caveats where GPS can be inaccurate for other reasons. But for the purpose of collecting enough data that I'm satisfied in looking at it, it's going to be around 2-3 min of distance.