Your justification in the first paragraph would work if not for the fact that even Hestia'd be aware of the fact that the weapon being in Bell's possession would greatly increase his chances against the monster. It is a very obvious thing. Like you very well noted, the time between him locking her up and then leaving to fight the monster forms a hole in that justification.
Let's just say Hestia panicked and just didn't know what to do and somehow forgot about handing it over. But even if we take into consideration that both Bell and Hestia were highly panicked when they parted ways, a thing that could exponentially boost Bell's chances of survival wouldn't just be forgotten about that easily. It doesn't make much sense, really. As for Bell not listening to her - they did spend enough time before parting ways for her to easily hand it over to him without an issue. Yet she didn't. True, he probably wouldn't have paid heed while running off but handing it over surely wouldn't have been a problem during those few moments they spent together between the time Bell locked her up and then ran off?
At this point I'd just chalk it up to plot-convenience rearing its ugly head, tbh.