This is just my personal experience and opinion. I hated the flowers, because they were white sympathy flowers and I had been anticipating congratulations, new baby type flowers with cheerful colours and balloons and congratulations on the new baby cards. Getting home from hospital seeing bouquets of sympathy flowers just rubbed it in for me. Second time around, we knew to tell people with the announcement - no flowers. We bought ourselves bright flowers. I realise others may take comfort in flowers. For the boys' funerals, we had bright coloured flowers.

When an angel is born here I congratulate the new mum and dad on their precious bundle. Because they did meet their baby, she did give birth, she deserves congratulations as well as tears.

I'd say Congratulations on the birth of baby [Name], I am so sorry s/he couldn't stay. Wishing you lots of strength and support for the coming months and years.
I'd write it in a colourful card that's neither a sympathy card nor a new baby card. And send it with bright pink or blue or yellow flowers to honour the bright light that shone too briefly.

If you have time, google Carly Marie. She and a friend have started a range of babyloss cards.

As for mentioning the baby. I hate it when people don't mention my boys, even to just say, sorry for your loss. The family are in a world of pain. Acknowledging that goes a long way. People can learn to live with feeling awkward. It won't kill them. Baby death is part of life. The baby is a real person who deserves recognition.