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.
Best Answer
The best way is to interrogate a single monster over and over again and reset, though for the sake of the collect all ship parts trophy you should keep any other ship parts collected in their place and go to capture an additional monster.
From my experience in the game, the X-Dimension chests were slightly more likely to have them or other high level ship parts, but they are also far more difficult to collect the part from. If you get a different Flonzor series part, you're on the right track and you just need to keep iterating.
You should absolutely save before interrogating, as reloading a save from after interrogating seems to have no effect on the item in the chest.
Finding the individual P Flonzor parts was actually not nearly as much of a challenge as getting the final body piece. It only took me about 4-5 hours of work to get all of them once I had unlocked all of the x-dimension levels.