I would like to stop by and say Kiddush for someone who is homebound and unable to say it for themselves. This would be on Shabbat morning after schul. However, I don't want to be Yotze from this kiddush, since I would like to be included in the Kiddush I will say for my family upon arriving home! Can I actually do this or am I automatically "yotzei"?
Yes, you may. See Shulchan Aruch siman 273 siff 4 'One may make kidush for others even though he is not eating with them, because for them it is kidush in the place they are eating. And even though the blessing on wine cannot normally be made for others if he is not drinking along with them, since this borei pro hagafen is necessary for kidush, it has the same rule as kidush hayom, and one can bless it for others, even if the one making the blessing does not drink
The reasoning behind this is the chidush of Arvus tells us that someone who is chayav in a mitzvah himself can be motzi someone else, even when the motzi, the one performing the mitzvah, is not intending to fulfill the mitzvah himself. Whereas regular birchas nehenin does not work like this. See Double AA's answer here https://judaism.stackexchange.com/a/19126/759
Extra credit: Reb Shlomo Zalman Auerbach writes in Minchas Shlomo 1 3 that he is uncertain if Arvus works in a situation where the one doing the mitzvah is not being yotzeh himself, and this is happening at a not optimal time. For instance on Friday afternoon where someone who was not mikabel shabbos yet wants to say kidush for someone else and not himself. The same goes for someone blowing Shofar after Alos Hashachar but before Neitz when the Tokaya is not being Motzei himself.
This is brought in Halichos Shlomo hilchos Tkiyas Shofar chapter 2 siff 12. See footnote #50 which mentions Rav Shlomo Zalman was lenient for a cholah or in a situation where the listener would not have an opportunity to fulfill the mitzvah later. That footnote also brings his son R' Avraham Dov Auerbach as saying the source of The Shlomo Zalman's uncertainty is not clear.
But if one does want to take this chidush into account, then perhaps making kidush for someone between Shacharis and Mussaf which is not ideal, as seen in siman 286 siff 2 being than one should not be eating a seuda then, might come into play.
There is no restriction on the number of times you make kiddush on a Shabbat morning because you are only reciting biblical verses followed by a blessing over wine that you have to make each time as it's a different drinking session.
Daytime kiddush is really a "procedure". i.e. when you have your first daytime meal you need to start with a cup of wine. Introduced by the G'mara. Later poskim (I have no source for this) introduced the minhag we have today of reciting some relevant p'sukim about Shabbat to make a declaration of "k'dusha" so it doesn't look like any ordinary wine-drinking.
Sufficient on Shabbat is just the verse starting "Al Kein Beirach" followed by the blessing on wine. On Yom Tov just the verse starting "vayedaber Moshe es moadei Hashem" will suffice to keep this "minhag".
There is no aspect of "Bracha L'vatola" for reciting Biblical verses such as these which is why you can recite them multiple times. (However for the B'racha on the wine it needs to be a separate drinking session, and you do of course have to drink, although b'di-eved it may be permitted to make the b'racha on behalf of someone else who is going to drink).
There seem to be 2 questions:
- Can I make the kiddush for someone else without being yotze
- Is this a problem if I want to make kiddush for my family later.
For the first one I am suggesting that to be not yotze you would not drink the wine (you'd recite the b'racha and the person you are making kiddush for would respond "Amen" and drink) or participate in a seuda (kiddush must be b'makom seuda). Ask your Rav before doing this if you want to try it this way as there are issues with either of these.
For the second one I can respond confidently that you can make kiddush for your family later even if you are already "yotze" due to the reasons I gave above.