I think Gideon was right about Isaac being a soulless husk, and Lucy only imagines he loves her (or anything). I'm not sure exactly why he isn't supposed to exist, unless perhaps Lucy was never supposed to marry Mike.

Three series of Lucy chasing Gideon could get boring, so hopefully there is more than that if a season 2 happens.

I think the show pretty clearly shows that previously, she didn't marry Mike. She instead married Ravi. So Isaac previously never existed, and she previously was a cop as well. Gideon makes mention that he's always caught by the same detective; Ravi mistakenly thinks that's him, but Gideon means Lucy. Keep in mind she's the one that caught Gideon this time round too; Gideon roundly beat Ravi and was leaving the scene.

Gideon is right about Isaac being "unbound," but wrong about Isaac being souless. I figured out early on that Lucy and her mother were experiencing flashes into their previous lives/timeline, but while for Lucy there's some incongruities in her life, like who she married, for her mother, her alternate timeline is one where she's dead, her loop should've ended decades ago. That shit must be terrifying, on some level knowing that you should be dead long ago. For Isaac though, it's even worse. He doesn't have any previous life to reference at all, even up to a point, all previous/parallel timelines are ones where he never existed. He doesn't know what he want, how to feel/react to situations because he has zero reference point. He doesn't even know what timeline he should exist in, since he technically shouldn't exist in any, so has difficulty keeping them straight, and can even slip between them.

I think Gideon may actually be scared of Isaac, what he represents, and may be oversimplifying what Isaac is, either to convince Lucy, or maybe himself.

• What's with Isaac's watch? Gideon seemed to think it was confirmation of something, when he found it after searching Isaac's room. Also, wearing it seemed to be what shifted Isaac to the "previous" timeline for that brief period he disappeared.
• Why did Isaac start complaining about being "cold" after his disappearance? In the voice message he left Lucy at the end, he says his dad Mike made him feel "cold," so it seems like more than just a physical feeling. Did Isaac start the fire at the end just to feel warm? Or was he trying to shift timelines again? Mike said he went back in to get Isaac, but couldn't find him.
• Why did Gideon wait so long in prison for Lucy in the last timeline? Why did Lucy want Gideon to change her timeline? Could Lucy detect Gideon's changes to the timeline even before he directly changed hers?
• Was Lucy at the end of the finale just remembering her past timeline, or did she "jump" to it? Did she die in the fire, and is now experiencing a future timeline? I suspect that this may be part of the "prequel' angle they mention in the season 2 announcement.