Thx for the TL
With how somber the story has been going, I don't think its gonna be some rough family drama between Tsubaki and the in laws. i don't think the manga gonna take a sharp twist like one of them cheated or something of that nature. Im more leaning towards Tsubaki's husband's family didn't all accept Tsubaki, and now that the husband died, the people who didn't accept her are just excluding her and hating on her.
Doubly so if they're the kinds that would see a gal as "not proper".
I think you're onto something overall, though, yeah. Making Tsubaki out to have had a rough marriage on top of her husband dying doesn't feel like it would be congruent with the rest of her character as it's been presented. And, I kinda doubt her husband's death will be anything super dramatic, either--either in a positive or negative manner (like, he didn't die saving children from a car crash, and she didn't murder him for insurance money, type things).
But I suspect that Umi will be a tie to Tsubaki's old life, and another way for Otakei to connect to Tsubaki across the span of time when they weren't in one another's lives.