Question

In the Talmud, no customs of mourning during the Counting of the Omer are mentioned at all. They are completely ordinary days. 
The earliest source is the responsa of the Geonim, where it says that the custom was not to marry during this period. Later they extended this to haircuts. Shaving is not mentioned anywhere at all. The Magen Avraham added listening to music. The question is: why do these things actually obligate us? Why should what was practiced in the time of the Magen Avraham, or in Safed, or in Krakow obligate everyone else? (As the Rabbi said, I haven’t been in Krakow for a long time.)
When does it stop being a custom and start splintering? Let’s say we accepted the Geonim’s custom not to get married—up to what point does every later addition that is written become binding on the public? (Obviously it isn’t binding in the full sense, because it’s a custom; the question is what the definitions of a custom are and how one should relate to it.) When is it possible to just ignore customs?