Does Adell (DLC) gain Big Bang in Disgaea 4

Not 100% sure on Disgaea 4, but they have been the same for Disgaea 2 and 3, so I'll go off that as it shouldn't have changed much.

There really is no value set on it and there are even levels within the ranks themselves, by this I mean 1 item will not 100% set them from Strongly for to love, but if we DID give them numeric values for the argument of this question, I'll try to illustrate below how killing would effect.

Say a senator has 500 rep points, a slightly interested item would be worth 10, interested would be worth 25, very interested would be worth 50, must have! would be worth 100, and not very interested or doesn't want would change in values depending on just how much they don't want it.

Killing would probably be around -200 / -250. If a senator loves you, and you kill him, he probably won't loathe you, but he'll most likely drop to around neutral / barely positive, which will take 2-3 or maybe even 4 must have items to make up in the long run.

Overall through my experience, never kill senators unless you're going for one those suggestions that has a 1-20% chance of success, anything higher than that I just save my game try it, if it fails, I reset and try again, but if it has extremely low chances I will bribe as many as I can and kill the rest, usually it doesn't have an overwhelming aftermath.

There's no apparent chronology outside of Disgaea 1, Disgaea D2 (Disgaea's direct sequel) Disgaea 2 (an apparent, sort of sequel). As you'll learn there are many, many alternate Netherworlds (and quite possibly different timelines). When Laharl, Etna and the gang visit future games they're generally doing so from an "alternate" netherworld to whichever world your protagonist happens to inhabit.

But for what it's worth [Disgaea endgame spoilers]

In the first Disgaea in the "Good" ending Flonne becomes a Fallen Angel, hence her existence as a Fallen Angel in most other games. I don't believe a reason is ever given for Flonne's sudden appearance as Archangel either.

Due to the above spoiler and various quotes from the Disgaea 1 characters (such as Laharl complaining about not being the main character in Disgaea 3) it does seem that Disgaea 1 happened before all other games and/or Laharl, Etna and Flonne show up in the other Netherworlds after the events of the first Disgaea.

Due to the reappearances of main characters as bonus/DLC characters in each successive generation it can be roughly assumed each game happens after the previously released games, however the remakes confuse this issue even more; certain Disgaea 3 characters now exist in Disaea 2 Dark Hero Days. But the Chronology is largely irrelevant outside of Disgaea 1/2/D2 if you consider postgame content to be non-cannon (since postgame + bonus characters is at best a time paradox/alternate universe funtime in most games), since Disgaea 3 and 4's stories exist completely separate of the D1/2 worlds.

I've only played the games and read the Disgaea 1/2 mangas though; I'm not sure what extended universe material is considered cannon or what material might give clues to the chronology.