My dsd starting talking in "babytalk" sometime after she moved in. (she was 8 at the time) Man, it was annoying! That being said, I'm sure it was her way of adjusting. Looking back, the more we got upset about it, the more she did it. I'm sure the whispering thing is the same. Its tough on them (you too!) and she's adjusting to you guys, new baby at mom's, etc.
So, that part I would ignore. No, I would probably whisper back. Maybe make some "tea" to "cure" her throat. Play into it. And have fun doing it! On a side note, one of my dss gets "ill" when he's stressed. So, I tuck him into bed with a damp cloth on his head and make him some tea and ask what else I can do to make him feel better. He's always better 5 minutes after he's tucked in.
Now, the eating part. I would let it go too. Theres so many battles ahead for you, let this one go. I'm sure she's been told (verbally or otherwise) that you are the "enemy", or maybe she just sees you that way herself. Who knows? But anyway, not eating may be her way of gaining "power" over you, and "winning over the enemy". I hope that makes sense. My suggestion would be to take her shopping, letting her pick some things. I wouldnt let her make the entire choice, just narrow it down, like pick a fruit - bananas, strawberries, or oranges? And letting her know what each one is full of - bananas have lots of potassium for....okay, I dont know why its good, but you get my drift!
Let her go to school hungry a couple of days but pack a muffin or something in her bag. Dont tell her, just do it. And if her bag comes home empty, dont say a word, just keep doing it.
Ask her to pick out a recipe from a cookbook. Maybe she can help cook? But nothing about eating or not eating. If she chooses not to eat, but must stay at the table (we do that rule too) then ignore the fact she's not eating.
I understand your concern about her not eating, I want to let you know its a very valid concern. Unfortunatly though, if you're stressed, then it adds to her stress.
I would also take out some "healthy living" books from the library and leave them around. Or read them yourself and tell the family what you've learned, as in "Wow! Did you know....blah blah blah". She'll indirectly get the info she needs.
In the meantime, maybe let her eat in her room? Take a snack to bed? Eat dinner in front of the tv? Whatever you do now doesnt have to be permanant. (We let our ds eat in front of the tv to just get him to eat, now he's older and understands the eat at the table rule.) Just some thoughts to get some food into her and ease your mind.
Well, I've gone on too long, good luck!
p.s. I just wanted to say good for you for seeking help, its hard, I know. You should be proud of yourself.