Is the zakat on a sum inherited by a merchant and invested in his trade added to the zakat on the original trade capital, or is its zakat separate at a later time? And does the merchant pay zakat on the full capital with which he started his trade, even if part of it was a loan, or does he only pay zakat on the profit he actually possesses after repaying the debt?

If you are able to distinguish each asset and its profit from the others, then you have two options. But if you cannot, then you must pay zakat (definition) on all of them at the end of the hawl (one year) of the first of them. You should deduct your business debt from your assets. If, after that, the remaining amount reaches the nisab (definition) and a hawl passes, then zakat becomes obligatory.