sdfhsdf ok this is a really good question, but also we should absolutely plot out more modern/reincarnation au stuff since we very conveniently write all three of them between us!!! 

relating this back to guinevere, guin/jenny is not the sort to be unfaithful by nature, the fact that she was unfaithful in canon has more to do with the matter of choice and the lack of it than it does literally anything else. that being said, to be honest i really don’t see jenny cheating on art. so if she and lance do enter into a relationship, it could only be before or after guin and lance were together. 

jenny IS drawn to lance, of course, in a way she couldn’t really explain without knowledge of what had come before, she just wouldn’t act on it.

that being said, there’s definitely the possibility there for her to end up with lancelot after art breaks up with her, particularly in any period of time between the break-up and her gaining back any sort of memories of their previous lives. while i think she truly does care for lancelot in this verse, i don’t think it’s the same way she feels about him in canon/legend. 

the result of this is very much up to plotting/interpretation/etc

and, of course, this time everything is different. she had chosen to be with arthur, she hadn’t wanted to break up with him, she’s very much in love with him in a way that’s both very similar to the way she had originally BUT ALSO different enough to make all the difference. and she is still in love with arthur after they break up

in a lot of ways, in fact, i think any of guin’s memories returning would be enough for jenny to end the relationship with lancelot, in much the same way that arthur’s death separated them at the end. this time, though, it comes with the realization of how unhappy she had been in that life, how few choices she’d been able to make for herself, how restricted she’d been, etc. and, hindsight being what it is, probably does have some sort of realization that choice/the lack of it played as big a part as it did. 

(which isn’t to say i think how much she loved lance in the past/could love him in these verse should be minimized, nor that making any choice would be easy, particularly depending on timelines)