I couldn't agree with more with the others who have commented. I am glad you contacted the rescue...had to do that once myself for the same reasons, and are satisfied with their explanation. Regardless, it is my opinion that they should have been more up front with you considering they knew they were adopting out a sick puppy. Courtesy is courtesy, and if they had been up front with you, you might still have/probably would have taken Momo anyway BUT KNOWING she needed medical attention immediately and what kind.
As for the previous owners...it is nice they gave you the insurance information...perhaps the word "nice" should be "appropriate" considering the circumstances. But beyond the 'final transfer of ownership' of Momo from them back to the rescue, I cannot see that they are owed anything more...and certainly not from you.
I'd express my horror at such a tiny puppy being on anti-anxiety meds but there are just no words fit to print in this forum...
...that aptly describe my thoughts on the matter...
...and thank you for taking her off Puppy Paxil.
Congratulations...and welcome to YT! I really do think Momo is better off in your loving care. I hope Momo recovers quickly from her illness, and am sending best wishes and healing thoughts for her speedy recovery. Thank you for giving her your best...and a truly loving furever home. I believe you will have loads of fun with Momo!