1. Enable the console and type in "runscript zz_seb_debug", which starts a dialogue.
2. Choose "Duty - Act 1".
3. Choose "Jump to location".
4. Choose "Chantry - Exterior".
5. Go to the chantry board and accept the quest "Duty".
6. Open the console and run "runscript zz_seb_debug" again.
7. Choose "Jump to a plot point".
8. Choose "Duty completed".
9. Open the console and run "runscript zz_dae_debug". <-- Note that there's another code here!
10. Choose "Area Jump".
11. Choose "Act 2".
12. Choose "Major interiors".
13. Choose "Chantry".
Now you will be in the chantry in "act 2". You will get the quest "repentance" and all you have to do is talk to sebastian to recruit him. He will act as if you did his quest in act one.
I have not encountered any problems with this solution