My partner's BM sent a complicated email earlier this year demanding that he take the kids shopping for her birthday because it was 'only fair' - this is the same woman who NEVER has them get anything for their dad for Christmas or his birthday. I don't care if she does or she doesn't - though for a woman who claims to value politeness so much you'd think she'd put some effort in on this one (and tbf her kids, esp OSS are very polite).

It's good if she has changed and be happy to accept the good things - and don't pick at them (it's easier to send cake, not quite as easy to send whipped cream and strawberries). Be happy she sent that message by text - you have it in writing!  But neither would I necessarily TRUST that she has changed. :-) 

The girl got her father a card and his favorite cake.  Why can't that be enough?  Why does it have to picked apart?

You did the same thing when BM and grandma gave Mothers  Day and/or birthday day cards.

"We did have some of the cake for dessert last night but if BM and SD REALLY knew DH's preference, they'd know he likes his angel food cake with whipped cream and sometimes strawberries."

What the what?! Do you feel like you're in competition with these two or something?! That sentence reads like a little kid who goes, "yeah, well, I'm MORE his best friend because I KNOW he likes smelly markers and not just regular ones!"

BM went and spent her own time and money to help SD pick up a gift and card for your DH. A gift he ACTUALLY likes! Even if it isn't perfect, that doesn't mean it wasn't a kind gesture, and thoughtful to boot. I'd be beyond irritated if my Dad's GF or my SF ever downplayed a gift I got one of my parents because it didn't meet their idea of what I SHOULD get them, and you'd blow your top if SD ever even made a half-second smirk at a gift you bought DH.

You can always think BM has an ulterior motive while not ruining a nice gesture for your DH. BM has done a few things that, in my mind, I am trying to figure out why she did it. However, publicly, I smile and am gracious. And if you think your DH isn't thinking this is strange, or that BM doesn't have ulterior motives, you're likely wrong. My DH is more willing to give BM the benefit od the doubt, but he's not stupid enough to think that there might not be a reason she is being nice. Your DH, even if he doesn't say it, likely thinks the same.

Disney is right; stop picking apart EVERY thing. Had SD done nothing you would be mad. Had BM done more to help SD, you would have been furious. If you keep putting everyone in lose-lose situations, they're going to walk away from YOU. This was a kind and thoughtful gesture. Leave it at that.

I love dogs, it will eat you alive trying to figure out the legitimacy of their actions. Their shortcomings are not your concern.

I would just keep on doing you, when it comes caring and doing for your DH, and let those two fall short. If I were you it would have gone something like this:

Me: “Are you going to leave the card like that (BLANK) for your dad?”

SD: “Hmpf, uh, yeah...”

Me: “Okay.” <walks away grinning> 

SD brought DH an angel food cake, his favorite, and a card.....   We did have some of the cake for dessert last night but if BM and SD REALLY knew DH's preference, they'd know he likes his angel food cake with whipped cream and sometimes strawberries.

I am sorry but to me this comes accross as you being dismissive and ungrateful.
SD did something nice, but is slammed because she could have done it so much better.

In fairness, how does one please you?
Considering this was done for DH?

Meh, Sd brought over the cake. If you thought it should include whipped cream and strawberries, you could have made a quick trip to the grocery and picked those items up to go with it. Counted it as your contribution. 

Your Dh is a father to two children this year, not just one. No, your baby isn't here yet, but baby is on it's way. What will you do next year? Have the battling which kid knows and loves Daddy the most competition? 

Anyway, I do hope your DH manged to stuff down some of his 'not perfect' gift and say thank-you to his daughter. 

When I stop to recall some of the 'gifts' your DH has given his daughter, I wonder what BM thought of him standing on her door step with a rose for a little kid? I bet she at times thought as she rolled her eyes 'oh, Dad, you put so much thought into just the perfect gift for your daughter'.