1) I don't agree with this complaint. As in all video games, there is tonal inconsistency in the Divinity games, but I cannot think of a way to 'fix' this without making things much worse. These jokes and light-hearted moments against the backdrop of an overarching threat have always been present in Larian games, and that's a good thing. Everyone remembers the existentialist skeleton in Divine Divinity's starting dungeon. It's funny when your character proclaims the Duke of Ferol to be the worst poet in the world, or when Billeh Ghar can't find a word that rhymes with 'orange'. And when I don't find something funny (e.g. the mayor being hard of hearing which means you have to repeat everything*), it's not annoying enough to take me out of the game.
2) I can kind of see where you're coming from with this one. To be specific, it's not that NPCs don't have personalities, it's that they all have the same personality: that of a perpetually smirking aristocrat. The writing is obviously extremely competent from a technical point of view, and the dialogue contains words and expressions that even some native English speakers might not be familiar with. However, the purpose of game dialogue is not to show off the writer's prowess, but rather to create compelling, relatable characters that fit in the game world and are internally consistent. I feel like this goal is not achieved when even random plebeians are spouting pompous lines that sound like they're straight from a Shakespearean play. I want to be able talk to actual human beings at least some of the time. Tongue-in-cheek caricatures are absolutely fine, but I don't think it's necessary for 98% of the NPCs to fall under this category.
Your 'solution' is silly, though. The better solution would be for some of the dialogue to simply be rewritten or toned down so that it is more in-character, which is absolutely feasible. The writer has a very impressive command of the English language, he just needs to learn to control his verbosity when appropriate.
3) I am not a game designer so I don't know what the best loot system would be. However, the ubiquitousness of 'junk' is a deliberate design decision on the part of the developers to make the world feel more alive and to encourage interactivity, similar to the old Ultima games. I fail to see the problem with that.
* actually, now that I think about it, this is a really lame trait to give to a character and it serves no purpose that I can think of. There must be a better way to convey that the mayor is supposed to be an absent-minded, narcissistic character.
