For Question #1, it's never too late, maybe explain something like "sorry I'm getting this out late, it's been crazy with the holidays, but here is a picture of Suzie in the cute outfit you got her!" and then go on to write all the nice "thank you so much" type lines. Becasue you're sending a gift card AND a picture of your daughter in the outfit it will be nice, becasue everyone likes giftcards and the picture will show that you really do like the clothes and were not just avoiding writing a note becasue you did not like them.
For Question #2, inquire if she still wants the clothes. Mabye she just forgot. If she seems wishy-washy, then take that as a hint that she wasn't really that interested, but maybe she has a lot of her plate and really did just forget and needs/will appreciate the reminder.