What you did was not inappropriate...neither would it have been inappropriate if you had included mention of her husband "We appreciate the two of you sharing our wedding day..." or something like that, but since you didn't go that route, it's fine...either way you showed appreciation for the gift and that's what the card is for anyway...her deceased husband (rest in peace) doesn't know or care so not mentioning him isn't being rude, it's being sensitive to the wife he left behind in thinking that mention of him might cause her pain...
Your MIL needs to STFU! It's none of her business. Sounds like your in for a fun ride with her.
Oh, yeah, Sending the thank you card to only her after the sympathy card was sent was the right thing to do. If you addressed it to her and her deceased husband, it would cause her pain, and would look really insensitive.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.