Oh. I'm so sorry to read what a terrible & testing time you've been through.
The most important thing is not to feel guilty about anything as it sounds like your instinct that something was seriously wrong was spot on all that time, it's just a shame it took so long to identify the probable cause of the problem.
I have consciously chosen not to feed cows' milk or any other cow dairy products to my ds as both myself & his father's other children have suffered from asthma & eczema which cleared up when it was eliminated from our diet. Of course it's not 100% certain that he would too but I just wasn't willing to take the risk.
As well as soya-based products (including yoghurt) you can buy a goat milk based formula called Nannycare which I would highly recommend. Both goat & sheep milk & their products have been found to be much easier to digest than cows' milk because the proteins in them are more similar to the structure of human milk & so are milder on the stomach. You can buy nice cheeses & yoghurts & fresh milk for cooking & cereal too (e.g. Sainsburys are now stocking lots of them & you can always request that your local store orders things in for you if necessary).
Don't be disheartened & make sure you try your best to relax & engender a positive attitude to food & feeding as your baby girl will undoubtedly pick up on any tension or anxiety on your or your partner's part & this could in turn have permanent repercussions. So: relax & take your time & give her space to go at her own pace & learn to enjoy eating & drinking & exploring all the wonderful tastes & textures that are out there waiting to be discovered!
Re: quantities. She has been used to only ingesting small amounts so her stomach will be small (I remember being shocked to read that a baby's stomach is only roughly the size of his/her fist!) but will gradually expand & gain greater capacity as she eats more regularly & in greater quantity. In the meantime you would do well to give her nutrient-dense foods so that she is still getting enough calories, vitamins & minerals to sustain good health & encourage growth.
Patrick Holford is an expert nutritionist & has recently written a book specifically dealing with the requirements of babies & young children. I don't know the exact title but you could find out easily by doing a Google search or checking on Amazon (if you're short on cash your local library should be able to source a copy for you but I get the sense from what you've written in your OP that it'd be very useful to have your own to refer to regularly so you can feel confident & empowered & take positive steps to give your dd what she needs & deserves). He writes in an easy-to-understand way & gives lots of examples too so don't be intimidated - just take a look & see what you think!
As for the Health Visitor, I think you have a right to see someone you trust & who you feel listens to you. I had an issue with one HV at my local clinic when DS was 7 months old & after expressing in no uncertain terms my dissatisfaction with the way she had dealt with both myself & my son I have since made it clear I only wish to have appointments with the other HV practicing there. I don't know whether you have more than one HV at your clinic but if not it may well be worth finding out what other options are available to you. If you feel uncomfortable enquiring directly at the health centre you could always ask your doctor to advise you where else you could go for support.
Anyway, I wish you all the best & hope at least some of this reply is helpful to you. Take good care of yourselves & your precious daughter!
MamaTama