Sorry to hear you lost one of them. . . It's so sad that your cousin would consider flushing a living animal! I'm happy that you stepped in to prevent that from happening. *sigh*
REALLY glad to hear you got the remaining one eating, though! That's a very good sign. Using a terra cotta pot to feed in is a great idea, you'll find that it won't take her very long to 'train' into going there for feeding. I've kept frogs with snails and have never had any problems, you'd probably be fine to remove the divider and leave the Apples in if you want to. . . though it might be a good idea to keep them separated for a while until the frog becomes used to the feeding routine. Typically, snails won't out-compete frogs for food like fish will, but it's always good to watch the frog eat so that you can be positive she is. I understand that this can be tricky if you have a nervous newbie, though. . .
As a general rule, mixing 'batches' of frogs without a QT period is exactly what you want to avoid. Just as with fish, the new addition could very easily bring in a pathogen that will harm the current, older resident. In this case, it is also entirely possible that the original frog was affected by stress caused by frequent moving and/or rapidly shifting parameters in a 1g bowl. Very happy to hear the survivor is in a 10g tank now. It's a much safer environment. ADF are perfectly content to live their lives out alone or with other frogs, but this doesn't seem in any way to affect their 'happiness.' I doubt that the lack of a 'friend' had anything to do with her refusal to eat, more likely to be stress related, they often will refuse to feed for a few days after being moved. Whats done is done, but I'd recommend you stick to this one frog for now, and not add another until you're more familiar with their needs and care. If in the future you decide to bring a buddy home for her, you will have to strictly QT for 3 months, being sure to keep all nets/buckets/siphons completely separate between the two tanks.
I don't know anything about the epoxy that you used, it claims to be non-toxic. Both frogs and snails are very sensitive to contaminates in the water, so I'd be looking further into that, too. That said, depending on how much you disturbed things while putting the divider in, it's possible that the haze is caused by nothing more than a simple bacterial bloom, which won't harm anything and should clear itself out after a few days. Definitely look into the epoxy, though. . . that makes me nervous.
The easiest way to tell the gender of ADF is to look for the post axillary subdermal gland - that's a super fancy word for what appears to be a small white or pink pimple. Males will have one just behind/beneath their 'arms,' pretty much in their armpit - females don't have one at all.
Good luck with the little one, and keep us posted on how she's doing, please!