You don't level up faster when solo. XP is not divided, everyone gets the same amount.
That is stupid.
There s no point in playing with less than a full party.
Not yet. There was a lone wolf talent in Dos2. I wouldn't be surprised if they implemented something similar in BG3. I haven't seen anything indicating whether or not they will though.
Personally, I like that all my characters are on the same playing field when they enter the party, as well as the fact that I get to choose how they level up past 1, regardless of what level I meet them.