passover - Why is hallel at the seder different from hallel during the year?

I know there are many differences between hallel at the seder and regular hallel.

At the seder, hallel:

1. Has no starting bracha (Orech Chaim Hilchot Pesach Siman 473)


2. Is said sitting down (Sefer Shibulei Haleket Inyan Rosh Chodesh Siman 373)


3. Is said at night (Hallel is supposed to be said during the day normally) (Bavli Megillah 20:2)


4. Hallel at seder is not part of the list of 18 times when a full hallel is said. (Bavli, Archin 10:1)

What is the reasons for these disparities/on what basis were they made in the haggada?