Always happy to answer!
I'll try my best to explain without spoiling too much. It is similar to what you just wrote: There might be an important moment where your MC has to convince someone, and if they are very dishonest, they might not succeed because no one believes them. On the flip side, there might be an important moment where your MC has to come up with a convincing lie, but if they've always told the truth, they might fail at that.
Now, what does that mean for endings? I think getting one of the 'peace' endings should have some difficulty to it, but the personality traits aren't the deciding factor. They might make it easier/more difficult, but they don't lock you out of anything. What's more important is the actual choices you make in specific situations.
To your second question, yes you can increase your relationship with characters even if you don't spend time with them, but it is much more difficult. There's three ways to increase (or decrease) your relationship with the characters: 1) dialogue choices in the main story (like when you first meet them), 2) spending time with them when given the opportunity to (like asking them to dance), 3) major decisions (like whether to set the mabs to lethal or not).
For example, it could very well be that you made some choices Vynn liked, and some they didn't like, so if you don't spend time with them, it might just stay roughly the same as it started. It's totally fine to just focus on one character if you want to see all of Maeve's scenes for example.