Dehydrated equals lethargic. Dehydrated also equals compacted stools, buildup of blood toxins, and a whole host of other bad things. If Mom is constipated she may not want to tell you but she won’t want to eat, either. At the very least, I’d try to get water into her. Dog can come in but you have to drink 8oz or 4oz. Time to take a shower/wash hair, another 4 oz. Go pee, another 4 oz. However you have to canoodle her into drinking water. Try to get her to get up out of bed for meals at the table. Once Mom’s hydrated she may want to eat a little more. Call her primary doctor. Explain in detail and ask for visiting nurse 2x/3x week for ‘fall’ follow-up. Don’t accept a rx for a antidepressant that takes 6 weeks to work - ask for xanax or similar - instant anti-anxiety. Also talk to the hospice care attached to the hospital. They may offer at least one or two home visits or can refer you to a private agency. If you can’t get a visiting nurse thru the doc, and can afford a few private care visits, call a private agency for a PA and a nurse aid (PA to rx IV, check for/remove impaction and the NA to change bed and gossip with Mom).
Bring activity into her bedroom. Music, card game even if she doesn’t join in. Take her for a car ride, maybe to see her house - does she need to retrieve a favorite blanket or throw pillow or picture? (don’t take the dog or she won’t want to go back to your house). Whatever it takes, eh? Try everything you can think of, don’t take rejection or mood swings personally, and if it turns out to not be enough in spite of your best efforts, you’ll have no regrets.
It seems you and I are on the same page. I brought a cat in for my Mom. It really made a big difference. That cat knows it’s job is to be her sidekick, and vice versa. The cat follows her everywhere, lays on he lap, sleeps at the foot of her bed.