The fact that you started out doing manga style you can use to your benefit. Anime and anime-inspired art styles are very popular right now, plus a lot of anime styles are simple enough you can draw a lot pretty quickly. If it really bothers you considering the time period you can change it, however I've seen period-inspired manga and anime work beautifully stylistically.
For example, the Interview with the Vampire manga above, adapted from a novel that took place in 1791, or the infamous Black Butler... Though I wouldn't recommend going that detailed for a single-person passion project.
As others said before me your comic would be fanfiction since it's not an official adaptation. As far as I know there are no problems with that so long as you don't try to profit off of it (and even if you did it's unlikely the creator would even find out).
For character consistency: I agree with what others have said before me, but I'd also highly recommend making a three point character turnaround!
Since this is a webcomic I don't think you need other views of the character; this should be enough to keep your character mostly consistent. Decide on a design and really stick to it, then make this turnaround. Keep the turnaround open at all times while you draw so you can reference every part of the design. Try to make it match up as much as you can. This will be slow going at first but over time the design will get into your muscle memory and you'll be able to draw without checking the reference much if at all.
For color palettes and clothing design: perhaps this is an easy way out, but have you considered making the entire comic in black and white or even sepia tones? Webcomics are a huge workload regardless of how simple the art style is and color is a whole other animal. When you have a story like this where you want to evoke that vintage feel, you might have an excuse to actually go black and white or sepia!
The variance in outfits you have for the characters is good so far. Like others have said before me though I'd also recommend making them more distinctive from each other; think of what makes the character unique and try to communicate that through the design. With the best designs, you can get a sense of their personality from a glance!
I hope that was helpful and if you have further questions lemme know!