Re: At wits end...
First of all, I want to say congratulations on nursing your child to the age of 21 months! You have done a great job so far, and I don't think for a moment that your child's issues with weight should be blamed on nursing- rather, I think that it's a good thing that you nursed him this long, because he'd probably have issues with eating even if you'd weaned him at 1, and this way at least he's been getting breastmilk all this time.
Breastmilk is really nutritious (See this abstract from the journal Pediatrics, and this article from Mothering Magazine), and considering the amount you are nursing, I'm guessing you are meeting all your baby's nutritional needs right now, even though he's not eating solids. I wouldn't be too concerned about the slowed-down growth, as even babies who are eating tons of solids tend to drop in the percentiles once they transform from sedentary babies into active, calorie-burning toddlers.
However, it sounds like this constant nursing and rejection of solids isn't working for you and your family. It sounds like you have a good thing going at home, with your mother's helper feeding the high-calorie solids. In your shoes, I would probably let things go on as they are for the duration of the vacation, and then go back to the way they have been once you are home. It's so hard to change things when you are on the road!
Also, it sounds like maybe you guys would benefit from some limits being put on nursing when you get back home. It's okay for you to say no, and to see if cutting back on nursing will encourage your child to eat more solids. Being out of the house, in interesting places, always helped my baby go longer between feedings. And I don't think you need to feel like you must NIP- public can be a good place to set nursing limits and offer snacks instead of the breast. Not because of privacy issues (Who cares, right?!! ), but because it might be an easier locality in which to enforce nursing limits.
Blah blah blah. Sort of a garbled post... I really just want to emphasize that I think you have done a great job so far and I don't think nursing into your child's second year is a bad thing.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"