There is no connection/link to a form field modified in Acrobat to the original form field that was created in Indesign. What you can do is maintain form fields in Acrobat, and then replace updated page content. This usually means moving and adjusting form fields back in Acrobat, but this task is no way as tedious as reconstructing form functionality from scratch.
Sorry, I don't understand this.
Can I create the form structure in InDesign, open it in Acrobat Pro, write all the calculation fields (many many) and then edit the form structure in InDesign - specifically insert a column or row - and then put it back to Acrobat Pro without losing - this is critical - my calculation fields/scripts, or any other field
Excel absolutely does not work since it erases everything and you have to create everything from scratch again. The excel spreadsheet I originally tried to use in Pro had 74 fields for data entry by the client, and 30 read only calculation fields. (6 columns, each with 5 formulas, all on one single page) When I was asked to insert a row, the re-import of the excel sheet (single page) wiped out everything and I had to do over again from scratch.
For the situation you describe in post 2, here's what you can do: Proceed as you are with adding calculations with forms in Acrobat. When it's time to update (i.e. add a new column of form fields), open and modify your Indesign document. Keep new form fields on a different layer. Then export a new PDF of just new form fields. Open original PDF and new PDF. Copy new form fields and paste/merge into original PDF, modify as necessary.
If you were to export modified ID document, you will lose all attributes (including calculations) of anything you modified in Acrobat. There may be third party utilities that could manage this task, but I don't know of any.
To be absolutely clear: you cannot roundtrip (reopen in Acrobat, then InDesign, then Acrobat, then InDesign). InDesign does nothing with PDFs, except export and place them. It is not a PDF editor. Placed files get visuals ONLY.