It is impossible to confirm whether the message has been seen (using workarounds) or not. However, it is likely possible that your message hasn't been seen yet, even if the account continues to be active.
The main reason is that some people may not check their inbox regularly. Also, if they've a lot of followers, your message might not be immediately noticed (or missed) due to newer messages.
Yes, it's possible that the person you messaged on Instagram hasn't seen your message yet, despite making several posts. There could be various reasons why they haven't read your message: Message Requests, Activity Status Settings, Message Read Without Opening. Considering the various possibilities, it's essential to remember that everyone manages their messages differently, and not receiving a response immediately doesn't necessarily mean they've intentionally ignored you.