I'm a former swimmer, runner, and triathlete. Everything that Cain says is entirely plausible. I am slightly bearish on her triathlon career, but it's likely reasonable for her to shoot for a career as a professional triathlete.
1) The swimming question depends a lot on her times. By age 15, swimmers likely know whether they are a D1 talent or not. If she was under 5:00 in the 500 free, she certainly could have gone D1 (although maybe not for the very best teams). Between 5:00 and 5:15, she could have gone to a mid-major team, but wouldn't have been a star anywhere. Between 5:15 and 5:30, I'm sure she could have found a spot at a small D1, but this is more the range of trying to score at a D3 conference meet. (Of course, the 500 free may not have been her target event, in which case, look up the equivalent power points). Finally, I wouldn't read too much into it, but it is a bit surprising that Cain didn't mention her swim times.
2) She can't get past her physical issues just by switching sports. The training is different, the body type is different and triathletes don't always peak at the same time as runners. But the feeling of being in peak form and firing on all cylinders is the same. Cain for reasons that have discussed ad nauseum is not currently at her physical peak. It's likely that whatever she does as a triathlete, she could have been faster had she switched at age 16.
3) I find that some excellent runners also make excellent swimmers, and some do not. Part of it is experience, but I find that the talent aspect of technique is orthogonal to running talent. It remains to be seen whether Cain has talent in that area. If she does, she could have a successful triathlon career, but if not she may struggle.
4) Cain will likely never be a monster on the bike, but it won't make or break her. If she gets to a place where her swimming is competitive and her running is close to what it used to be, she'll figure out the bike leg. If not, no biking ability will save her.
5) A good reference point for Cain is Gwen Jorgensen. Gwen was likely around 4:50-5:00 for the 500 free in college (solid D1, but not making NCAAs), and an incredible off-the-bike runner. Her bike leg was what you get when you take that outstanding of an athlete and have them work very hard, but I wouldn't say it was a particular talent of hers. She came to triathlon at a similar age as Cain, and after "retiring" from sport. Cain will likely never be able to run like Gwen, and probably not swim like her either, but this is the reason that it's foolish to count out Cain for any success.
To summarize, it really comes down to two things: Does Cain have talent as a swimmer? And does Cain's body still have the ability to be the same level of overall athlete she used to be? If so, she'll be very competitive, potentially on an international level. If not, she may struggle.