what happens if you'll manage to persuade them ?

They let you live and ask you to find their soul jar and destroy it ...no rewards

The reward for smashing their soul jars (found in the dark cavern where Tromdoy is) was 4200xp. Its my first run, I have no idea if there is an alternate route.

I ended up picking up the soul jars and bringing them with me after handing the singular one to the lady in the sanctuary. The remaining 3 after persuading with Str them that a 'fate was worse than death". They let me live, without attacking. I dropped the jars on the ground and interacted with them, smashing them. Each skeleton's spirit was sucked up leaving the bones/loot behind. I got 4200xp after all 3 jars were destroyed.

Using the jars on yourself replenishes 1 source point if you don't have any, then returns an empty jar to inventory. Using all 3 jars completes the quest giving 4200xp.

wish you got exp for leaving them there as well after seeing their memories, freed them for the exp but would not have done so otherwise

they are worth less exp if you kill them. It is better to do the quest and smash their jars.

This is an other quest which is sort of bugged as even when destroying the jars it can still complete with the description "we left without helping the necromancers". I acquired all of these before initiating the quest. Destroyed one of them right when I got it, destroyed Gratiana's during the dialogue with her and the remaining two during the fight with the necromancers (they had already respawned a few times before). You can destroy them during combat by dragging them to the ground from your inventory and shooting an arrow at them.

You can in fact use the soul jars whilst in combat. Drop each jar from your inventory then attack them, the undead will fall.