Well, I know that I am *always* sceptical of things until I have read enough about them to make up a good opinion, and when it comes to alternative medicine I am usually wary until I see results. In your case, I know if it was me, I wouldn't take her word for it, but use it as a starting point. As in, take one of the things that she has said, like citrus, read up on the signs of the allergies, and start eliminating it. Watch for any improvements. Then move onto the next, etc.
As for the MPI, you are right about rashes and things, but one of my sister's LOs (all 3 were milk allergic or intolerant) still has a milk allergy at 8 years old, but it did take my sister a while to realise it because his rashes are sooo subtle sometimes that she hadn't noticed them enough to make the connection. She notices it more in his temperment than anything, which is one of those things that only a mother would notice and that a lot of doctors would brush off as being one of a million other things, iykwim?