Sometimes a child can swallow and can eat but the effort, co-ordination and strength required exhausts them and uses up the energy that they would have gained from the food.
It sounds like your DD is doing all that she can in the form of vitamins, fish oils, procal, etc., and it still isn't enough.
The charts, although a nuisance for most NT children, are actually there for a reason. You don't give your GDS age, but presumably he is past the age where milk is his main source of nutrition.
Additionally, children do most of their growing and strengthening during sleep, so if he is not a good sleeper, that will have a knock on effect because he is using energy at a time when he should be conserving it.
A PEG isn't a sign that your DD has done badly, or your GS. It is a sign that conventional feeding is not supplying him with enough calories to thrive.
It simply isn't logical to say 'well if he was 2lbs heavier at birth he would be on the chart.' If he was, he might be on the chart, but it is part of his overall condition that he wasn't 2lbs heavier at birth, and most children that are born at 4.5lb jump up the centiles as they grow to reach a normal weight.
Sorry, but I do think that sometimes doing to best thing for the child means getting past your ideals as a parent/grandparent. It isn't about either of you, it is about him. How would you feel if you were running on empty all the time? I wouldn't like it.