Would Eris have been into Elain if she were the bait?
if eris knew elain was a seer—or even suspected she had power from the cauldron like nesta—absolutely yes. but i'm not sure that he would be as interested in elain off rip.
i'm very much of the belief that eris pursued nesta as he did in acosf because he wanted the power he knew she had. he knew she could be an extremely valuable asset against his father. if the night court wanted to dangle her in front of him like meat before a dog, he would play the part; he would sink his teeth in and bite. then nesta was sharp, and smart, and he had the fleeting thought—oh, you are not like the others; you could be fun to play the game with. his want of her was self-serving, but there could have been something between them, eventually, because his interest was piqued.
with elain—things are different. iirc she never exposes her abilities to anyone beyond the ic because no one realized she had them; they all just thought she'd been brain-scramblied by the cauldron. i'm sure plenty of rumors circulated through prythian about this, stemming from the distant observations of servants and lesser fae and commonfolk. (whisper networks my beloved!!!) among them, i think there were some voices speculating that elain was damaged in some way by the cauldron, that it took from her as it did that mortal queen—she isn't seen around as much as nesta, she doesn't attend the meetings, etc. i don't think eris takes any rumor at face value, but he also isn't assured of her value in the way he is of nesta's.
then he sees her dangled before him: lovely and soft, choked up by night court black but blushing pink, nowhere near as convincing as her sister would have been in another time, in another universe. her pulse flutters wildly at her throat, and she is hardly able to hold his gaze, but he can tell that she has been coached through this—told to bat her lashes; to affect submission like the nc thinks he likes, though that doesn't seem much of a put-on; to act flustered by his presence. she presents much softer, more docile, more pleasant than nesta; she does not play the game very well. his interest is not piqued in who she is but rather what she is hiding underneath.
while they dance, he wonders after the nc's intentions in using her in this way. if she is as soft as she feels in his hands, why would she be chosen and not the other sister? is she expendable because the cauldron indeed took more than it gave from her? no, he has to remind himself—they don't treat their own that way. are they expecting him to want her, or did they offer her as an affront, hoping to rile him into some sort of reaction? are they dangling her like this to steer his eyes away from her because she does have great power, and they want him pining after the other sister instead? he can't be sure—not unless he bites, which, after weighing everything, he doesn't see the harm in. and so he sinks his teeth.