Sephardi Wine vs. Ashkenazi Wine?

Question

Aaron: I have a couple of questions to ask of you or your father about something I was told; namely that there were problems in Eretz Yisrael with using certain kosher wines for Kiddush and Bircat HaMazon at a Seudat Mitzva. 1. Is this problem related to the amount of dilution permitted by the Rama as opposed to the Mechaber? 2. Are there any kosher wines currently sold in the US that do not follow the Beit Yosef, and should be avoided by Sephardim? 3. In cases where we travel and am away from home on Shabbat or Yom Tov, and our hosts use wine prepared according to the Rama's opinion, should I: make my own kiddush on my host's wine, be only yotzer to my host's kiddush since it may be with a questionable wine, or should I only drink wine with an acceptable dilution? One of my teachers taught me that if I knew that I was eating at the table of a fellow Jew who is Shomer Shabbat and Yirat Shamayim, I was to accept their word as to the kashrut of their foods, drink, etc. in order to not show a lack of respect. 4. Does the scenario I mention in #3 apply to purchasing and eating challah as well? In Miami and Caracas, for example, the bakeries bake different types of challah for each community. Thank you for your reply - Kol Tuv! Jonathan