I don't see how this is such a hard Psak.
There are different ways of understanding Sefiras HaOmer and therefore there are different Minhagim that come out of these different understandings.
1) The Talmidim died until 33 or 34. That being so, one must wait to get a haircut/shave until Lag BaOmer by day or (Sefardim) 34 by day.
2) The Talmidim died 33 days, from Pesach until Shavuos. That being so, one must keep 33 days of Aveilus and one can be Miktzas HaYom KiKulo. Some therefore keep from Pesach til Lag BaOmer and some keep from Rosh Chodesh til Shavuos (but one can't burn the candle at both ends - R"CH til Lag B"O).
According to R' YYF, if one is getting a haircut/shave on Erev Shabbos, he has already used his MH"K up and therefore when Lag B"O comes he must wait until the day ends to get his full 33 days. It's not the biggest Chiddush and very understandable.
May one get married on Friday? No. Because pashtus MH”K isn’t a factor here, it’s a heter specific for כבוד שבת.
Additionally, its something not mentioned by all the achronim, you’d think we’d hear about it, after all the Remah goes second half, and he’s the one introducing the heter.