The line is accurate, but confusing. The number of points is the same across the board, 75 * number of players, but lower-difficulty AIs have slower rates of accumulation. Unfair and up don't get a cheat, so they stay at 1x multipiler. It's the lower AIs that go slow.
As for negative points, it's just a display issue. All the math works with the negatives as you describe, so if you have -3 from missions and -1 from military and -1 from race relations, you do need +5 to get to the actual 0. I'm fairly indifferent on actually displaying the negative as opposed to leaving it at 0. It would make it easier at a glance on the new Diplo screen, but hasn't bothered me much to actually suggest a change during the beta
neither did it bother me, because it had no effect. whether you had 0 or -3 was irrelevant, it meant that you would be be finding an ally with this player anytime soon. now, with the diplomatic victory, it does have an implication, since you want to find out what is giving you so much negative points. those would be a good canditate for wiping out or having wiped out eg via bounties or pirate attacks. I loaded up my present game and tbh, I only saw that I had + 3,6 or so relations in total. therefrom, one gave me + 6,5 and another one some 1.5. so, what about the other two? no freaking idea. the game was nicely along and there were so many factors already working (nice!) that it was just plain hard to calculate the actual number and see where I got the remaining ~ 4.5. I also had trouble determining why some other ais had so good values.
all I am saying is: this is now relevant information and it should be presented as such, i.e. easy to find and process.
oh, and I love how this forms a fairly interesting interplay with some diplomacy research. points bonus or the removing of racial penalties are now not 'just' to make cease fires and alliances easier. they are also a quite direct path to alternative victory.
and because I am in suggestions mode: if there is some content work again, a consideration could be to allow research that improves envoys diplo bonus generation. for example, one might have research to allow for higher per planet vaules than 1.5 or faster accumulation. if the devs ever feel like adding stuff.