Calculating tax

The most obvious thing is to ask the tax authorities in the country that you are reporting to how they would like the figures to be presented and what supporting documents they require. It can hardly be a new problem.

Just apply the numbers pro rata according to how many months you need. If you need to provide back up documents, then the copy of your extrait you provide will show a higher amount paid, but you're only going to claim a pro rata amount for the appropriate number of months. That is perfectly acceptable. But I agree with BECASSE above, just ask the tax office.

Just send them the tax documents that you receive in Belgium at the end of the year. That should be fine.
And/or call the tax office to be sure.