I have to wonder if all these characters getting introduced in short order will be actual romance candidates for our ML.
Because there's every chance that they're not; they're simply various people in his life (or, in the case of the Landmine Girl, soon to be in his life) that create either tension, or present obstacles, or opportunities for him to grow through his troubles and come out a better person on the other side.
The other mangaka could be a rival, but also someone who pushes the ML to improve his craft in a way that our FL can't from a technical/industry-insider perspective. She might be not romantically tied into the story whatsoever, though she might present a red herring of sorts, as she relates to the FL.
And the same could be said for the younger in-law; she might present something similar in terms of narrative obstacle, but focused on the FL as the sister of her late husband. She might be romantically linked to the ML either--and I'd actually be dismayed if she were, given her relationship is more with the FL than anything else, and I think I'd expect her to be upset at the FL for "moving on" with the ML, instead of continuing to pine away as a widow or something.
Either way - I'm gonna just hold to there being no harem tag until I'm shown otherwise. I'd really like to see a series that doesn't have every female character who walks on-screen inexplicably fall for the ML, anyway.
Thanks for the TL.