mentch, pouring into other peoples’ cups after drinking is not a shailoh because kiddush has been performed. You need the cos not to be pagum for it to be kosher for kiddush – the kiddush is done, and the people are drinking from a kol shel bracha after the fact. It is also not a chiyuv to drink from kiddush for those who are yotzei, but it is the minhag to do so and some say it might be a chiyuv at night, but me’ikar hadin it isn’t necessary.
re, drinking first then dispensing: chazal say that we’re not supposed to drink from a cup and then give it to someone else, as the machlah can be on the cup (chazal clearly understood about germs) but the exception to this is wine, because alcohol kills germs.
Nafka mina would be grape juice.