There are many details that can prove this, such as the fact that Mera's mother always told her to stay away from magic at all costs, but this very emphasis is problematic.
This kind of binding without any weight not only failed to keep Mera away from magic, but instead made her even more curious about magic. This was a kind of psychological reaction, but it was extremely typical and easy to exploit.
Like a provocation.
And reality has proven that.
Mera not only came into contact with magic, but she came into contact very deeply.
But she was very careful, perhaps because of her intuition, so Mera never let herself fall into true exhaustion.
But all of this, the purpose of keeping Mera alive longer was only so that she would become a more durable shield for her own mother.
Kain could even make a more malicious guess, that Mera's father meeting her mother was not a coincidence, but was premeditated.