As well as the impact (sorry!) of white foods etc., there is actually a pretty strong link with gut issues, so please don’t worry about not being able to get her to eat.
It’s more important, I think, to reduce drama around food, and let her eat what she wants for some time, before starting to reintroduce new foods, and then just putting them on the table etc. there are loads of websites/books on this, and I’m sure you could find a thread or two on here.
Meanwhile, yes, people with ASC often have hyper mobility, low muscle tone, and part of this can be slow digesting/peristalsis. As they would say in the adverts for Yakult, slower transit time 
.
Stay with the lactulose etc for now. I know it goes against the grain to give it to a 2 year old long term, but as you have seen, impacted bowel etc., can be extremely serious.
This is a long walk, not a sprint, so don’t try and know everything all at once. Deal with each issue as it arises, and that way you will have a huge brain full of knowledge over time.
It’s tough, but as pp says, there are all sorts of fb pages, and so on.
I really recommend some of the threads on mn. I was very concerned about the autistic adults who report awful mental health problems as a result of be therapied constantly as children to try and make them more “normal”.
As a result I always look for threads/groups that include autistic adults, and always always always, assume that my child is reacting to discomfort if they are being “difficult”, and so far that has stood me in good stead.
