If I would like to fast on erev rosh hashana and mincha is usually davened in todays minyanim together with maariv in shul is it permitted to fast until z’man kiddush, or should one take a water bottle to shul so as not to go into Yom Tov still fasting (but it might already be too dark)? Furthermore, for one whom a fast doesn’t bother too much is it ok to go into yom tov fasting (or Shabbos too)?
For Rosh Hashanah, there is no problem of fasting until kiddush, and this is the practice of many tzaddikim.
When fasting, one cannot drink water until nightfall, and one cannot drink water after nightfall until kiddush (Shulchan Aruch, Orach Chaim 271:4; 289:1), so that if one is fasting, there is no other option.
Even for somebody for whom a fast is not particularly bothersome, it is preferable, but not forbidden, to avoid fasting on Friday and erev chag. This means that if one has the option of choosing this day or a different day, one should avoid Friday.
However, if one is fasting a certain number of consecutive days, and one of those days is Friday, then one should fast on Friday (until kiddush on Shabbos), and not doing so will break the consecutive days.