We are beginning siman 23. The Chayei Adam writes that if Shimon is in one house, and Reuvein is in another house, and Shimon hears Reuvein make kiddush, if Reuvein has in mind to be motzi Shimon, Reuvein is yotzei even though they will be eating separate meals. The Mishnah Berurah adds that even though the wording of the Chayei Adam appears to indicate this is bedieved, it is actually muttar lechatchilla. Thus, for example, if a neighbor is unwell and homebound, a neighbor can be motzi him in kiddush through the windows or the like, as long as they both have in mind to be yotzei.
In siman 24, the Chayei Adam writes that once a person accepts Shabbos, there is an issur to eat or drink until one makes kiddush. This issur applies even if a person accepts Shabbos from an early time. If a person did not accept Shabbos early (although, as we have learned, it is preferable to accept Shabbos early in order to fulfill tosefes Shabbos), the safeik Shabbos generates a chiyuv kiddush and an issur to eat beforehand.
The Chayei Adam clarifies that although one cannot eat before kiddush, if a person wishes to clean out their mouth, it is muttar, even though they may inadvertently swallow some water.
We have learned that it is preferable not to begin a meal close to Shabbos, but if a person began the meal beforehand, they do not have to stop until Shabbos begins. Once Shabbos takes effect, the chiyuv kiddush takes effect and they must stop eating until they make kiddush.
Regarding making new brachos after making kiddush, the question depends on whether we consider the meal as interrupted or not.
There are few cases where this question applies. One scenario is when Purim falls out on Friday. Some suggest that one can make a purim seudah which will go into Shabbos, and make kiddush again when Shabbos begins. If one wishes to use this suggestion, they should speak to their Rov for more details.
Summary
One can be motzi another in kiddush even if they are in a different mokom, provided they both have kavannah to be yotzei.
Once a person accepts Shabbos–even early Shabbos–there is a chiyuv kiddush and issur to eat anything before kiddush. Cleaning out one’s mouth is not considered eating.
There are cases where a person can begin a meal before Shabbos and make kiddush in the middle, but one must speak to their local rov about this question.
