A quick note on "pace" - pace means nothing.
Pace is calculated with the recorded distance and time. If you walk two miles in 30 minutes, you have a pace of 15 minutes per mile, no doubt you will get credit for near enough the full 30 minutes - all is well.
Say the same walk has a point of interest. A pond the dog likes, a statue, something like that. You take an extra 15 minutes to enjoy this, and add another 0.1 miles to your journey (say you're wandering up to it, or around the edge of it, something like that).
Your average pace has dropped to >21 minutes per mile. Is it fair to no longer include the 2 miles you walked at 15m/pm because there was a slow stint in your walk? Of course not. The watch still has your heart rate recordings for this time, it (should) know the pace you were doing for this time, perhaps you don't deserve the full 45 minutes, but you certainly still deserve the original 30.
If pace really is the measure of whether or not something constitutes "exercise" I think that's a shame. The amount of technology built into the watch to track things like that, and you bump into an old friend or discover a hidden gem and forgot to hit "pause" and your whole workout is wiped out? Shouldn't be the case.
I do not believe pace alone is the issue. And believe AW to be "smart" enough to figure things out just as you say.
For example, I will often do a full hour workout out the gym. However, the "circles" rarely give me a full 60 min exercise and it makes sense. Why? My hour "at the gym" includes a 5 min warm up. I imagine may warm up, especially the first couple minutes is not intense enough to register as exercise.
And the opposite happens. If I go for a 60 min run, I sometimes get credit for 62-63 min. Again makes sense. Why? After my run, I walk for 5+ min to cool down. The first couple min are likely still registering as exercise due to my movement, HR, etc.
I actually find the exercise ring the least useful of the three. I'm already in the habit of exercising 6-7 days a week and nerves less than 30 min. Mostly 45-90 min and occasionally 2 hours. Since at this point, you cannot change the exercise ring goals, I find it rather useless.
After almost four weeks, I'm finding Move much more useful, motivating and accurate. And I'm still trying to find a challenging but achievable calorie number. Remember Move and Exercise are part of each other. Calories burned during exercise count towards move calories. Plus you get credit for calories burned that did not count as exercise.
But to your example, yes you should get credit for any minutes that meet the criteria of exercise. Apple even notes this by saying you don't need to do it all at once. Indeed, I have noticed getting credit for a few minutes of exercise here and there even though I haven't done a formal exercise yet.
If you have doubts on it working, try different situations. Preferably controlled where you know the variables. Then if still in doubt have Apple check it out.
But I'm very, very pleased with the fitness aspects of AW. Both in motivation and accuracy. Yes, I wish exercise had more options (like setting higher minutes or raising the threshold of what is exercise). But those may come in the future.