I think we touched on this yesterday – did you see my reply? I offered some tips on how to request a call from Amazon SC and to request they stay on the call with you. You mentioned you had tried to get hold of Amazon support to walk me thru this but been a struggle. What was the struggle? – was it onnecting with them or following their live support on the phone?.
For a partial upload, the main mandatory fields are the first columns PLUS the field you want to change:
Item Type Keyword
Product ID Type
For character count – yes, some categories have reduced word count. Unfortunately, there is no work about, so it just means every word needs to count and with a superior focus on Benefits vs instructions for use to lists of features. I did notice that your Style guide download for Grocery. Was a Version: August 2016. So check with SC help.
Category Nodes. Yes, you can change the Node once listed. You can also do this on the same flat file. You do want your category to be within Grocery/Food Cupboard/. To see which is the best one, have a look at the top sellers in the category and select the one that best suits your product and group of competitors.