Jim COULD still be alive... if he can storywalk, or be brought along
First: Suppose Jim can storywalk. He's the gene carrier, or whatever. Or Julie can bring people with her.
There's no continuity of scenes in terms of dialogue or characters between the penultimate and last scene we see Jim in.
There's no reason they have to be Jim in chronological order.
All that's required is that at some point Jim storywalks back into the "present" to be killed by the MIY.
A potential series of events:
- <off screen> Julie storywalks back in time and grabs Jim
- <off screen> Julie explains what's happening and convinces him, tells him about lake of tears
- <off screen> Present Jim goes into hiding
- <off screen> Jim storywalks back in time knowing he will die, and walks pensively to the RV
- <on-screen> Julie goes to Future-Jim/MIY confrontation, and tries to save future Jim
- <on-screen> Future Jim dies, Julie disappears
- <on-screen> Present Jim talks to Ethan (NOT a ghost). Tells him about lake of tears
- <on-screen> Julie learns how to storywalk
- <off-screen> Present Jim learns how to storywalk, and helps the gang "from the dead"
- Still to come: Ethan tells Julie about lake of tears
- Still to come: Julie finds present-Jim, who tells her what needs to happen
- Still to come: Present Jim + Julie go back in time for the MIY ruse
Meta-narratively this also makes sense - last season ended with a big cliffhanger around the MIY and Jim, with a surprise twist - it was future Julie all along! But the fandom immediately cottoned on to that, making it a rather lame twist.
Having Jim also be a storywalker in the same scene really would make people go back and re-evaluate everything happening on a re-watch.
This would also very neatly explain why Jim is a ghost. Because he's not actually dead (yet), he's real-Jim. He just knows he has to die at some point.