Ok, but even in this case, you're ignoring major flaws in this entire idea, even if the whole theory of a "core" and a "seep" is true (which it almost certainly isn't, as that mechanic hasn't existed in the past as I recall, and the graphical difference in puddles just seems to be the texture stretching out as it expands).
For starters, there's no easy, safe, reliable way to actually kill a whelp far enough inside an existing puddle to have the edge of the new whelp puddle not stretch over the edge of the old one without actually standing in/on the edge of the puddle yourself. Even then, the inclination is to skirt the edge of the puddle, but since it always expanding, we're talking low single-digit yards in distance between safely doing what you intend and suddenly taking 20-60k damage puddle damage depending on the Twilight Spit count at the time.
More importantly, you aren't acknowledging the impossible geometry of putting whelps in a valid location within the puddle without you yourself stepping in it. Due to the radius size of the initial puddle from a whelp, I'm quite certain there's no way to put a whelp deep enough inside an existing puddle while meleeing you without also being in the puddle yourself as I mentioned above, but even if it is possible, we can be certain that location will have to be as far into the puddle as possible: the radius of the initial puddle or slightly more. However, because the movement path of a whelp is identical to that of the tanking player, with no other variables, the line of travel is always a straight line, which means you can never place a whelp inside the puddle without also stepping into it yourself. If you skirt the edge, the whelp too will skirt the edge, and even with minor NPC jostling that occurs while tanking, it won't be the desired distance or location at all -- they will be off to your side toward your starting point before travel.
Typically in a straight path you get something like this:
Now, even if you could slightly cut the puddle with something like Heroic Leap or a Shockwave stun and quick run around, you'll never cut into it enough to actually move to the opposite side of the puddle to create your ideal puddle drop location. Also, the puddle size is ever-increasing, meaning the amount you could "cut" to force whelps in where you want will be ever smaller.
Then, due to the "core" vs "seep" theory, if a core actually exists, this means whelps must be killed in new locations around the puddle each time. This requires a leap/lockdown movement for every single one, yet the DPS requirement is quite high and won't allow for such time wasting things. Whelps have 200k health, and they spawn every 50 seconds, so this requires 20,000 constant DPS from the moment they spawn to the moment the next set responds. Reality is 5 seconds are spent gathering best case, so we're talking upwards of 22k DPS required, which just isn't happening if you're killing them off one at a time in select locations, yadda yadda.
No. Whelps spawn every 50 seconds outside of Phase 2, so the only time for natural stack reset is during Phase 2 (which was already mentioned in my first post). You'll never be able to drop a stack in Phase 3 naturally since the debuff lasts 60 seconds. Dropping the stack will require immunity from Vik as mentioned, or stopping whelp tanking.
Ultimately, I don't understand the obsession with these ideas. Puddle growth is only an issue if they are single-killed as seen in the videos that do it that way, and the 10-stack double-kill completely eliminates the danger of them by keeping everything quite tightly packed. It seems you're overly concerned for no reason and we've spent far too long debating it -- this is a convoluted solution to a problem that doesn't actually exist, based on a theory that isn't even proven. Not trying to be a dick (more than normal), but whelps are the least of our worries and we have a very workable strategy already setup.