(1)- We are accustomed- the implication is that it is only a custom. And the custom relies that since there is a scriptural proximity of eating Matza to the Korban Pesach. As it says "And he shall sacrifice the Pesach to Hashem, your G-d" (Deuteronomy 16:2) etc. and (immediately after it says) "Seven Days you shall eat on it Matzos" (Deuteronomy 16:3) etc. Thus we say like with the slaughter of the Pesach it should be after the 6th hour (on the eve of Pesach), so the making of Matzos of the mitzvah should be after the 6th hour. But in a strictly legal sense the Poskim establish that there is not to worry about this because the main scripitural proximity (i.e. a hekeish where closely placed laws in the Torah are derived to have similar related rules) is said regarding eating Matzvah, on the 1st night that one should be careful to eat before Chatzos (halachic midnight) like with the Korban Pesach like is elucidated later in Siman 477, but not regarding the making of Matzvah. Therefore, after the fact if one bakes matzvah before the 6th hour even a month or two before Pesach, and does it for the sake of the Mitzvah of Matza, it is fitting (to fulfill the mitzvah on the 1st nights by the Seder).

(2) Mitzvah Matzah - that it is necessary to him to fulfill his obligation on the two first nights but other matzos we are not accustomed to be careful in this (stringency of preparing on the Eve of Pesach).

(3) Until after etc. - Look later in Siman 460 Paragraph 3 that it is necessary to be careful when kneading during the time of the prohibition of Chametz. Behold most Jews are not accustomed to be careful even in Mitzvah Matza to knead on the eve of Pesach except for a few of them. I searched in the Later Authorities and found in the Bigdei Yishah (commentary on Shulchan Aruch) a reason to protest the custom because there are many opinions that even on the eve of Pesach a little bit (i.e. the prohibition of Chametz which begins on the eve is regarding even a very minimal amount of Chametz causes a large amount to become Chametz) like was eludicdated earlier in Siman 452. And this is hard to be careful that he does not find even a little Chametz in the wheat that they start not to sprout. Therefore we bake before in order to nullify (the potential) Chametz. And in the Maamar Mordechai I found that he writes, and this his language, "Nowadays many are lenient in this matter because it is tough besides for a few that are careful in this to knead and bake after the 6th hour the Matzos that one fulfills in them on the two nights. And so is fitting to do."

(4) 6th hour - behold the Chok Yaakov sides with the Bach that after six and half hours specifically; however, in the Magen Avrohom and in the Elya Rabbah, they side like the Shulchan Aruch that after six hours. So is the p'sak in the Bais Meir, and similarly is implied in the Biur HaGra. And one should know that from a legal perspective (i.e. as opposed to stringency considerations) it is permitted to bake Matza on Pesach; rather, those particular (in mitzva observance) are accustomed to bake everything prior to Pesach, that if a small bit of Chametz gets mixed, it can be nullified as opposed to on Pesach that the prohibition includes a small bit. And obviously when baking before Pesach even if there was in it a small bit of Chametz in the flour, it was already mixed and nullified, and permitted afterwards to bake the Matzos on Pesach; since, there is not becoming Chametz after baking. And there are those men of action that are strict on themselves, and they do not soak nor cook Matzos on Pesach from a concern that perhaps a little flour remains in the Matzos within that did not knead well, and by soaking it will become Chometz. See the Sharei Teshuva Siman 460 that from a legal perspective, there is not to worry for this; since, an establishment of a prohibition we do not establish (i.e. we do not assume a prohibition without cause) and especially nowadays that the custom is to make them very thin (i.e. the Matzos). Nonetheless, one who is accustomed to this stringency, do not abandon it, see there that he details in the matter. Write the Poskim one who bakes on the night of Yom Tov Mitvah Matzos should not bake except for what is necessary that night, not for the 2nd night. However, if he baked (some matzos) while still day, on the 1st night, he can say I'll eat today (i.e. the 1st night) hot bread (i.e. freshly baked Matzos). Then he will be able to bake other Matzos on the 1st night (to eat on the 1st night itself), and those that he baked while still day he will store away for the 2nd night. Write the Later Authorities that initially it is not worthwhile to bake on Yom Tov in order not to come to complications in separating Challah on Yom Tov. Similarly (there are issues) in pounding the flour, washing the utensils, that the children shouldn't fall asleep, and that one not come to eat the Afikomen after Chatzos.

(5) Knead Erev Shabbos - that is to say that not like the Stict to bake on the Night of Yom Tov as was explained at the end of Subparagraph 4.

(6) After the 6th hour - as a remembrance to other years.