As I wrote in the previous post, there are two events that we are commemorating by mourning during the omer, and they seem to have happened on different dates. Rabbi Aqiva’s students died in the first part of the omer, while the Crusaders reached Ashkenaz in late Iyar. Depending upon which is event you consider primary, I could see justifying observing either part of omer.

Akiva Miller found 12 different methods all told for observing the omer and posted them to mail-jewish (back before Avodah existed). Looking over his sources, I made slight modifications to produce the following.

1. The Ari, as per the Shaarei Teshuva 493:8, who is quoted by R’ Eider, “Halachos of Pesach” vol II, pp 330-331. It’s also mentioned by R’ Blumenkrantz, “The Laws of Pesach – A Digest”, 5753/1993 edition, pp 17-2, 17-3, as custom 4a. All of these use the phrase “ad Erev Shavuos” implying that one may get a haircut erev Shavuos during the day. They make no mention of not having to observe Lag Ba’omer.
2. Same as #1, but ends the morning of the first of the Yemei Hagbala.
3. Rama 493:2, as per Mishnah Berurah (MB) 493:6 and Bei’ur Halachah “Yeish Nohagim” (BH). In the BH, it is the second custom listed under the first opinion. It’s mentioned in the Igros Moshe (IM; 2^nd custom) and R’ Eider (ibid; B), quoting this Rama, Blumenkrantz (ibid; custom 1) and R’ Aharon Fleder in “Moadei Yeshurun”. In the last two sentences of the teshuva, R’ Moshe says that the Rama was giving this opinion for Sepharadim, disagreeing with the Shulchan Arukh (SA; next).
4. SA 493:2, as explained in the MB (ibid) and BH (ibid; 1^st custom in the 1^st opinion), the Aruch haShulchan 493:4, Igros Moshe (ibid; 1^st custom), R’ Eider (#1) and R’ Blumenkranz (custom 2). [R’ Ken Bloom added in a comment (below) that this “is also the opinion of R’ Ovadia Yosef, found in Hazon Ovadia Hilchot Yom Tov, and in Yalkut Yosef.”]
5. BH (ibid; second opinion). He takes this as an explanation of #11, and not a separate custom. IM (6^th minhag), quoting the Magen Avraham (no reference), however, R’ Moshe holds no one follows this custom. Of the 39 days, 6 are going to be Shabbos, leaving 33.
6. IM (5^th minhag), quoting MB 493:15 from Siddur Derech haChaim. This is the custom of Frankfurt.
7. Magen Avraham, as per Beer Hetev 493:8, MB 493:15 (who also quotes Chayei Adam), AH 493:6, IM (4^th minhag), R’ Eider (C, “There is…”), R’ Blumenkrantz (custom 3b), R’ Felder and is the custom in Elizabeth.
8. Be’eir Heiteiv 493:3, quoting Or Zarua
9. R’ Blumenkrantz (custom 3a)
10. IM (3^rd Minhag) quoting the Rama 3 and MA 5, and R’ Felder.
11. Rama 493:3, first opinion, as explained by MB 493:14 (2^nd half) and 493:15 (1^st sentence), Beer Hetev 493:7 (quoting the Bach), Beiur Halachah (ibid; 2^nd opinion), R’ Eider (C: first 2 paragraphs).
12. Akiva lists the first two opinions of the Beiur Halachah as (what I rendered into) #4 and #3 above. The BH doesn’t believe anyone holds by the third opinion, so it isn’t listed.

R’ Moshe lists 6 customs, given here as no.s: 4, 3, 10, 7, 6, and 5 (in that order). R’ Moshe says that the Rama paskened against the first of these (i.e. #4), and that in practice, no one follows the last (5). He shows that the other 4 are variations of the same minhag, and therefore one can switch among them without annulling the implied vow of having followed one particular custom.

The two most common minhagim are probably #3 for Ashkenazim or #4 for Sepharadim (which differ over Lag ba/laOmer) and either #10 or #11 (which only differ by whether one mourns the day of Erev Shavuos).

(Key: █ – mourning entire day, ▀ – mourning only at night)