You've gotten some good suggestions and you seem to be, doing all the right things.
I do think that the lack of poop could be due to the decrease in food going in. I definately notice that with Emily.
One thing to remember, as hard as it is, and as "dismissive" as it may seem, is that it's not at all unusual for a preschooler to go a week or longer with very little interest in food. My non-cfer was a TERRIBLE eater at that age, and still often needs to be encouraged to eat. (She's away at camp this week and I'm certain she's eating almost nothing!)
All kids have their likes and dislikes, and all kids go through periods where there just aren't interested in eating.
You are right to keep offering lots of choices and to try to find things that interest her. Have you had her help you make a batch of cookies or let her spread her own peanutbutter on a sandwich? EMily almost always tears into something that she got to pick out at the grocery store (Unfortuntely, that usually IS fruit snacks...*sigh*).
I hear your concern. I worry about it too, and I celebrate every portion of a pound that Emily gains!
You're on top of it, though, and you won't let it get too far, so try to step back from it and just wait. I know that's hard.
Let us know if you find a sure-fire trick! We all need them!