Washing Out a Cup Prior to Kiddush

I recently was told of an interesting minhag. That is that prior to making kiddush, one pours water into the cup and then that water into the bottle of wine or grape juice. Then, one pours the grape juice into the cup.

I was thinking of various bases for the minhag. Two I rejected and one seems probable.

First, the two rejected:

1) There is an idea of pagum on wine which was sipped from, but you can add wine to the cup to make it non-pagum, and then pour it back into the bottle. But then it was clarified that this was done before any wine was in the cup.
2) There is a Talmudic basis for diluting wine, either with 2 or 3 parts water to 1 part wine, but that was because they had thick, strong wine. And here, the water was poured first into the cup and then from there into the wine/grape juice bottle.

It seems likely that the basis is:
3) Halachically, the requirement of washing the cup is only to make sure that it is clean. Therefore, an unused, clean cup suffices without washing.(M.B.173:1) Kabbalistically, there is a
stress to always wash out the cup before use.(Kaf Hachaim 173:4)

(All this, BTW, is based on Berachot 51a (see here)):

Rabbi Yochanan said: We only have four: rinsing, washing, undiluted and full.
Rinsing - is done on the outside.
Washing - is done on the inside.

Therefore, the custom developed to pour water into the completely clean cup, before kiddush. What to do with this water? Perhaps because there was nowhere to pour it when indoors at the time of kiddush, someone decided to pour it into the wine bottle. After all, perhaps the glass would otherwise be full of water. Or at least be too much dilution, whereas in an entire bottle, it would not dilute the wine so much. Or perhaps for some other reason. Regardless, it seems an extension of a practice which is based on some kabbalistic source, and is not really in its base form halachically required.

For whatever the reason, it might pay to think through doing this when a guest in someone else's house. They might be OK with it, but on the other hand, they might get miffed at your diluting their whole bottle of wine or grape juice, for something that is after all just a personal minhag. And if the purpose is really to clean the glass, and the glass is not entirely clean, perhaps they might not want whatever dirt or germs in the bottle. And an additional problem -- while the alcohol in wine may prevent spoilage, apparently pouring back grape juice, or any liquid, into a bottle of grape juice causes it to spoil more quickly.

At any rate, an interesting minhag.