I'll give that topic a look-over (read-over)... I remember making a topic around the completion of the Kickstarter and it not getting much response
In an older system, that would mean balancing each encounter individually, leading to most power-players going for the most XP, or some original sollutions giving less or no XP at all. Some other examples of "how not to do it" is my most favorite game, where an entire level is lost if you make peace with a gang, rather than just murder them all. Even though a lot of the game is about conversation over combat, that segment actually picking the peace option harms you bigway, as do many other segments ingame. D:OS has somewhat the same burden where it offers many opportunities to solve something, just to reward the combat-heavy the most and leaving all others to dry.
To answer the questions:
1) It can. An objective is just what a developer has set it to be. This should allow easy control over the development of characters and on-the-drop modifications, compared to for example nerfing a skeleton's XP by 25 which affects all and has a large effect throughout the entire game, less predictable, more time required to apprase how the balance has shifted.
2) A thing noticed early on in Pillars of Eternity discussions about this is that gameplay type it's impossible to detect "sneak past"... it's just impossible to actually setup. A more realistic goal is to set "get to point X" and then reward that instead, wheter you got their peacefully or by force. Same result, but actually possible from a development standpoint.
3) If you got the diamond you got the diamond. While in a kill XP system indeed it's possible to finish the quest then squeesh more XP out by killing everyone (especially systems giving bonus XP for peaceful sollution then murdering them anyway getting their kill-XP aswell) in such a system that wont have any experience-benefits. There may of course still be many valid reasons for people to do so (roleplaying, revenge, other side-quest, suddden plottwist etc.)
4) Yes. Though I personally would oppose to such quest-design
And of course it may open up other potential "rewards" than just XP. Like saying if you leave the ancient cavern with the relic after doing the speech check, a band of cuthroats is waiting for you to reward you for you honesty with some good old fashioned combat. Since it doesn't reward XP, it doesn't overpower the player picking that option, nor is it a "good, I got waylaid, some more XP"... it's some good old fashioned "shit, I got ambushed, I hope this wont cost me" (I do hate how most ambushes or negative side-effects are pretty much welcomed by players due to XP).