That's not how SUMIF works. SUMIF is for tables, think of a spreadsheet for example. It says something like "If column A equals X then sum the value from column B".
But in your case, you are wanting to get the value of the checkbox and sum it, and it's not in a table. This is complicated by the fact that Forms is setting the values to start with V_ if they are a number.
I think your easiest solution is to use several IF statements that check is the box is marked and populate a value, or a zero if it's not checked. And add those together. Like this:
=IF(Interest_Rate_Values.V_9,9,0)+IF(Interest_Rate_Values.V_12,12,0)+IF(AND(Interest_Rate_Values._other,Checkbox._other_data<>""),Checkbox._other_data,0)
The downside with this, is that the other textbox doesn't have the ability to be limited to just a number, and if you put a letter in there, the field with the formula will show a formula error. You could get around that by making a checkbox that says Other instead of the built-in other field, and use Field Rules to show a number field when it is marked - instead of the built-in other field.