I have never tried one of those, so i don't know how well they work. From the video, it looks like you cram it up in there pretty good. How would you get it out of there if you needed to? It (also) looks as though you have to remove the light fixture to thread the cord through the plugs' hole, and re-pull the light.
But i am probably wrong there...
What we do is use a product called A+B epoxy putty. Its a bit pricey, about 35.00, but we have used it for over 25 years, with very satisfactory results. Since you are letting the pool drain down, i would let it go till it stops, then not fill back up for the "plugging".
- Remove the light from the niche and set it on the deck.
- Prepare your patch (A+B).
- Cover the area where the cord(s) come through the niche, be sure to make a good seal (obviously). Next step is most important.
- Do not reinstall the light at this time. Re-installing the light now would more than likely move the cord in the conduit, and since the patch is still soft, open up your patch, and resume the leak.
- Come back tomorrow and re-install the light.
Note: This epoxy has no problem with a wet environment, so no worries if it gets wet.
We have had very good luck with this particular procedure. The trick is to use enough epoxy, but not too much, remembering that some day, you may need to remove that fixture and don't want to hinder the removal and re-installation by an excess of epoxy patch that you couldn't get out of the conduit. More or less cover the opening, and perhaps push a little into the conduit, maybe 1 inch but that's all!
I would hold off on doing anything though at this point, until the pool builder addresses HIS leak! You/I (as the customer) would only do this in the event the pool builder abandons you. However, it sounds (from your posts) that his is willing to make good.
Good Luck