(There are potential spoilers for his canon here.) UPDATED: May 29, 2020
1. What information can Elias simply know about your character if he concentrates on them? Secrets they don't want exposed, people they want to protect, memories they want to forget, crimes they haven't been caught for...things like that.
He calls himself Solus zos Galvus (or just Solus Galvus) but he's far, far more ancient than he looks. Honestly, there isn't much in the way of secrets with him - just things he leaves unsaid if no-one asks. Elias will certainly know that his true title is 'Emet-Selch' and that the form he wears is not his own. Rather, it's one that he's forcefully taken and shaped for his own use.
(There's...a lot I can elaborate on but it would involve spoilers from his canon, and I'm not sure which parts would interest Elias.)
Currently he's suffering from recurring nightmares thanks to having shared a tale from his past with Jon, the Archivist. Elias may find that interesting to know. As of mid-Dec, 2019 he has managed to free himself from them with help from Lilith.
NEW: As of the end of May (or thereabouts), Emet will have found a semi-solution for the two nightmares he fed the Archivist a few months ago thanks to Aziraphale.
2. Will Elias be allowed to implant memories into your character's mind? These memories will always be true and can be good or bad. But once he gives them to your character he can never remove them. (Example: He knows character A's friend was tortured but A does not. He then gives A memories of character B being tortured and how it felt and so on.)
Would like to discuss this scenario if it ever happens, but will put a tentative yes for now. He has so many memories already what more could you possibly give him.
3. Is he permitted at any point to create a contract of sorts to bind your character to him? Please note that this means he may not tell your character that this has taken place and that if he comes to harm so will your character. This will also be a separate contract from the ones used in Duplicity.
No, but that's only because he's already beholden to another deity in canon and said deity provides specific protection against compulsions like that. Elias is sure free to try though... He wouldn't be the first here. /eyes Jon
Felih removed his tempering around the middle of March. So Emet is now vulnerable to Elias' machinations.
Solus zos Galvus/Emet-Selch | Final Fantasy XIV
UPDATED: May 29, 2020
1. What information can Elias simply know about your character if he concentrates on them? Secrets they don't want exposed, people they want to protect, memories they want to forget, crimes they haven't been caught for...things like that.
2. Will Elias be allowed to implant memories into your character's mind? These memories will always be true and can be good or bad. But once he gives them to your character he can never remove them. (Example: He knows character A's friend was tortured but A does not. He then gives A memories of character B being tortured and how it felt and so on.)
3. Is he permitted at any point to create a contract of sorts to bind your character to him? Please note that this means he may not tell your character that this has taken place and that if he comes to harm so will your character. This will also be a separate contract from the ones used in Duplicity.