Is it permissible to invest money with a jeweler such that I receive a fixed 20% annually from the capital, with forgiveness whether the profits increase or decrease beyond that? And should the percentage be of the profit or of the capital?

Paying money to the jeweler can be done in one of two ways: Either through Mudarabah (profit-sharing partnership), which means you give your money to the jeweler to invest it for a known, common percentage of the profit, and in this case, any loss is borne by the capital and not by the Mudarib (the working partner) unless he transgresses or is negligent. Or through Musharakah (joint venture), which means you contribute a part of the company's capital, and the jeweler contributes another part, and in this case, profit and loss are shared according to the proportion of each partner's share in the capital.