Could the person receiving the letters have been involved?

It’s possible in the abstract, but the details here don’t establish it. Being away when her husband died, or surviving a later trap, can look suspicious in hindsight without showing she planned either event. The theory needs evidence connecting her to the letters, the weapon, or the setup, not just a motive that seems to fit.

I’d break the theory into separate claims: she had a reason to want her husband gone, she wrote or arranged the letters, and she had access to the weapon and trap. Each needs its own support. Handwriting familiarity or possible access to someone’s belongings is not the same as proof of authorship or involvement.

The fact that the letters focused on the woman rather than the more prominent person doesn’t tell us much about the writer’s gender or motive. Harassment can target someone for all kinds of reasons. That pattern might be worth asking about, but it can’t carry the case for a particular suspect on its own.

For me, the biggest issue is how many assumptions the theory stacks together. It asks one person, possibly with help, to orchestrate the letters, a death, and a trap while making another person look responsible. Without a concrete link between those events, I’d keep it as a possibility rather than treat it as the strongest explanation.