“As a BTW – Do you think the original Heter Chassidim had on let’s say Zman Tfilah is still relevant today?”
The reasoning for allowing davening later than zman tefillah is that in our time it is not possible to daven with the proper koved rosh and kavannah before zman tefillah, and that if one or the other has to be given up, it is better to give up the zman then to give up the quality of the tefillah, especially being that it is permissible to daven until chatzos, and hechsher mitzvah mitzvah, and other rationales.
If this was true at that time it certainly true now in an even greater way. But since I do not make up things for myself, the way that I, a Lubavitcher, knows that it is permissible is because The Rebbe says so, in addition to the fact that Lubavitcher Rabbonim say so, and my Mashpia says so, just the same was as anyone in any given community knows how and what to do based on a chain of tradition within their community.