I was wondering if you swaddle? My baby is 5 months too and likes the woombie or the miracle blanket. Actually he hates them but it's what soothes him to sleep. She could try swaddling him, feeding, and then patting his back while she lays next to him like you would for nursing on the bed (Even better if your baby will use a paci, mine won't). My baby seems to really respond to this if I let go and relax with him. If I'm keyed up just waiting for him to take a nap it's like he senses it and stays wide awake. If I close my eyes with him and snuggle he usually succumbs. Now the trick is actually staying awake long enough to sneak away! We also have a loud fan that provides white noise so the sounds of my movement don't wake him.
Another idea is to take him on a car ride for a nap. Or put the car seat in a grocery cart and wheel him around the store. I know the car seat isn't babywearing friendly but if she's babywearing other times I'd say he's getting his snuggles in.
Maybe the carrier isn't a good fit for her? The Ergo is known for being easier on the back. I also like wraps. For my back problems I have found a back carry to be more comfortable.
As for those hammocks, the kanoe doesn't look like it would snuggle a baby enough. The other two look like better options. Let us know what you do. I'd love to know how they work as they are a really beautiful idea!