I saw this late but these are my theories.
Even if Mielle turns the hourglass she won't be able to turn back time since it's her gift/power given from God in the story so only Aria is able to. When the prince saw the hourglass nothing happened too. It would've been nice if they had a scene about that though like you said.
I agree with the guns since there's none at the time.
Guns were in use as earliest at the 13th century and by the 16th century it might’ve been very normal for noblemen to own or use a gun. By the setting of this drama it might’ve been late to early 18th-19th century so guns were in use. Though it doesn’t delve into much details but it can be understood that they used guns during that time of history
It's nice
There's some plot holes, one that I still can't figure it out is when the younger sister goes after and grabs the hourglass??
There's no continuation. The plot hits the wall there.
There's also the concept that as she travels back in time, her body grows quicker. It's initially followed, but it's soon forgotten.
And another thing that threw me off the fantasy was the episode where there was a panel of the FL and her mother where they were holding guns, insinuating the survival they had to endure, but it didn't made a lot of sense to me because the period time of when the story is situated doesn't have guns but swords or other weapons of sorts.
That really stuck with me :/
Also, after the 100th chapter, it gets so unnecessarily confusing, thought in the end, it's able to give an explanation. It was prolonged for no need.