One possible scenario in a fanfiction about Jimmy's old lover could be that she returns to Downton Abbey unexpectedly. Maybe she has changed a great deal since they last parted. She could be seeking Jimmy out either for revenge or because she still has feelings for him. This would create a stir among the servants and the family alike, as secrets from Jimmy's past start to surface.
Definitely fiction. Downton Abbey creates a fictional world with made-up characters and events, although it may draw inspiration from historical periods and social contexts.
Well, the old lover could come back pregnant, claiming the baby is Jimmy's. This would cause a huge scandal at Downton Abbey. Jimmy would be in a difficult position, having to decide whether to believe her and support her or deny the claim. The Crawley family might also get involved, as they value their reputation. And the other servants would surely gossip about it.
Yes, Downton Abbey is based on a novel. The TV series takes inspiration from various literary works related to the period and social settings it portrays.