A possible solution: Add three additional attributes for Array (Multi Flexi) (Text) (numbers only)
1 it might be worth making the question Array (Multi Flexi) (Text) (attribute: numbers only) into it's own question type

2 add the following attributes:
Show totals

• row = total for each row (an additional column)
• col = total for each column (an additional row)

• both = totals for both columns and rows (an additional row and column)

  store totals in results

• 1 = store column/row totals in results

  Grand total

• 1 = show grand total only
• 2 = store grand total in results (do not display)
• 3 = show grand total and store in results

Java script would calculate totals for columns/rows and would be able to hand negative numbers and floating point numbers.

for visual example see
http://apps.acu.edu.au/~evanw/LimeSurvey_custom-q-type_array-mulit-flexi-text-numbers-only.html

Please ignore styling as this should be done with CSS.

I can help with the HTML side but can't help with JS, DB LS admin stuff.