I don't know the answer to some of these questions because my expertise is not in Britain or Russia, but I did want to point you towards a wonderful primary source - the Journey of Anthony Jenkinson, which is a reprint of a number of first person travel accounts from people engaging in this trade through the Muscovy Company. You get a strong sense of just how much these people were throwing spaghetti at the wall to see what would stick. For instance, Jenkinson's mission is to travel all the way down through Russia to Iran, where he's supposed to present a letter from Queen Elizabeth to the Shah; but when he finally arrives, after getting spectacularly lost and almost dying several times, the letter is written in Latin, Italian, and Hebrew, so no one in the Shah's court is actually able to understand it!

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Queen Elizabeth conducted a lot of her trade diplomacy through letters to rulers of this kind, which were intended to allow a company to set up a monopoly on trade; so this was really a top down situation, not a merchant to merchant free market exchange. This makes the need for interpreters a little less pressing because you only need one to arrange things at the official negotiations and then the rest sort of runs on rails. She corresponded directly with the Russian Czars for fifty years through letters which were brought to the court by ambassadors and translated there. Russia traded mostly in furs, tallow, flax, wax, hides, ropes, etc. (basically raw materials) and the English traded fine cloth, tin, sugar, lead, brimstone, etc. The trade could never have been a state secret because Russia was simply not remote in the way that the East Indies were to Europeans. The Dutch were already happily trading with Russia before the English got in on it.