1. Yes, all of the Divinity games have been using a classless system, but [in my opinion] players should be rewarded for sticking with one particular tree. They should be rewarded with a very good and unique skill that is on top of the talent tree [because people who mix skills tend to pick up only the best ones from each tree which is kinda unfair].
In what respect should this be "unfair"? No Divinity game has ever had a PvP mode, and neither will D:OS have one, as far as I know. There's absolutely no need for characters to be (roughly) equally powerful, regardless of which skills they've learned.
In all of the previous Divinity games, the weakest characters were created by newbies who accidentally spent their points on low levels of the weakest skills from all trees. If anyone, it should be those characters who are getting a boost.
The classless system encourages playing unique characters and experimenting with unusual combinations. If choosing skills from different trees automatically meant that you'd miss out on "a very good and unique skill", many people would probably stick to the role models like in most other RPGs where you don't have this choice. Larian would be encouraging players not to use the freedom in skilling that has been an important feature of all Divinity games.
Let everyone choose the skills they want, whether these are the most powerful ones from each tree or the most powerful ones from one tree or the ones that suit best to the character the player has in mind. If specialization gets a unique reward, versatility should also get one.
