When Avraham Avinu went to the Akeida the Satan made a river that he shouldn't be able to pass by. There are a lot of stories about Gedolim, if they wanted to do something and they saw a Siman from heaven not to do it, for example, Birkat Cohanim in חוץ לארץ. Why didn't he take it as Siman? The Rishonim say the command was not clear what he should do it. If so I didn't he take this as a Siman from heaven not to go?
ב"ה Shalom Your question is an encompassing one which touches upon elements of faith in the Torah. Though, I don't think I can cover the issue completely in the scope of my little corner in the website, I will touch upon some aspects. First of all, I don't think we can ask questions upon the actions of Avraham Avinu as described in the Midrash (תנחומא וירא סי' כב). The Midrash describes the uncompromising Mesirut Nefash of Avraham Avinu to fulfill the word of Hashem and to withstand the challenge Hashem placed before him. Even in regard to the actions of Eliezer, the servant of Avrahamin finding a wife for Yitzchak Avinu, there are questions in regard to his actions if the they didn't fall under the category of the prohibition of "nichush", of acting upon the basis of omens. The Rambam (רמב"ם הל' עבודה זרה יא ד) considers Eliezer's actions as "nichush" and sees them as an example of a forbidden action. However, the Ravad disagrees with the Rambam as says that his actions were permitted, because the actions that the Torah forbade are omens with no logic. But the signs Eliezer made for himself had logic to them. So the question is how to deal with "siman from Heaven", of the example you brought in regard to Birkat Kohanim by Ashkenazim outside Eretz Yisrael. You are referring to the what is told , (ערוך השולחן סימן קכח, ד ,במעלות הסולם ,ד' ט"ז ע"ב) about the Gra and Rav Chaim Volozhin, one was taken to jail and the other's Beit midrash went on fire because they wanted to initiate Birkat Cohanim in Chutz la'aretz. Before their attempt of initiating Birkat Kohanim there was already a psak of the Rema in the Shulchan Aruch (קכח:מד) not to do Birkat kohanim except on Yom tov. So, not doing Birkat Kohanim for whatever reason, was just going back to the original custom. It's not that, due to some sign from Heaven they stopped doing a mitzvah which people had been doing for generations. No doubt whatever happened with the Gra and Rav Chaim Volozhin had its reverberations for generations to come, but that doesn’t bring us any closer to understanding what brought things about that it was decided not to initiate Birkat Cohanim in Chutz la'aretz as opposed to the ruling of the Rema. In any case, an individual cannot decide to fulfill a Mitzva or not on the basis of an omen and the example of the mesirut nefesh of Avraham Avinu illuminates our path for generations to come. As we approach Chag Matan Torah, we can Daven "גל עיני ואביטה נפלאות מתורתך." All the best שבת שלום וחג שמח