Boy, your story sounds way too familiar. I was on TPN with my second pregnancy, and had hyperemesis with all three. Of course, I kept trying to follow doctor's orders, didn't know I had a problem with gluten yet, so I tried eating crackers all the time (gee, wonder why I couldn't keep them down...

).
My first was born very small (induced 3 weeks early because I also had pre-eclampsia), and I had quite a bit of difficulty keeping his weight up, but it was because he had a hole in his heart. I'm sure your pediatrician would have caught that by now if your child had it; our pediatrician caught it at 7 weeks.
Did getting more aggressive with solids make any difference?
I was kind of a maniac about good nutrition for my first; first food for him included rice cereal mixed with my milk, sweet potatoes, pears, broiled salmon, and soft tofu cubes. He didn't get cheerios or crackers or things like that til he was in day care at age 2.
How is going gluten-free going for you? Are you noticing any improvement in yourself? I've read several times now that pregnancy/childbirth can trigger previously dormant celiac. It can also trigger thyroid problems (which are often linked to celiac anyway)--has your doctor bothered to check your thyroid levels?
This is probably stretching things a bit, but did your baby start crawling already? My babies all continued to grow taller but lost a bit of weight once they became mobile...Did your baby continue to increase height and head circumference? Are other developmental milestones on track?
But something about all you've written still strikes me as not adding up; I can see why you are worried. I wish I had an answer for you. Hopefully, it'll be something as simple as eliminating gluten from both your diets. (And be aware that there are a LOT of gluten-containing foods that you'd never think had gluten, like soy sauce, rice krispies, corn flakes, deli tuna salad,deli roast chicken, rice crackers, potato chips, "lite" ice creams--the list is unbelievable, and it took most of us MONTHS to figure it all out!)
One other thing--vaccines. This is a very hotly debated topic, and I'm not telling you to avoid all vaccines forever. But I do recommend you do a lot of research (www.nvic.org is a good start), and that you not let your child receive any more until everything is back on track. And even then, I would only do ONE vaccine at a time, as there are NO studies showing long-term safety of multiple vaccines. I recommend holding off on the MMR until she is at least school age, as there is quite a lot of evidence that the MMR is linked with autism and celiac --there was a study done by Andrew Wakefield that showed the MMR in the lining of the gut of a huge percentage (something like 24 out of 26)of autistic children with major digestive problems (probably celiac) years after it should have been gone, and the group of non-autistic children with digestive problems did NOT have the MMR in the lining of their gut. Note that I said "linked with;" I'm not saying that the MMR CAUSES autism, only that there is evidence that it, along with celiac, is involved somehow with autism. I'm sure many people will jump in here and say that their child received the MMR and was just fine, etc. etc.
When all is said and done, vaccines are
your call, no matter what the doctors (or any of us) say.
Anyway, I hope somebody comes up with something helpful here (the renal tubular acidosis sounds like something definitely worth investigating), and please keep us all posted, okay?