One of the only places I've seen recent Black Speech discussion is this thread on the Orklager forum (visiting in Chrome lets you auto-translate the discussion from German). Near as I can tell, they're a band of Orc-LARPers who typically use the Rukh Nûlûrz dialect (which is a close enough relative of LOS to steal ideas from, imo). The thread is about as active as this forum which is to say "not particularly", but you may find something worthwhile in it.
For inventing words, the advice on the Dictionaries page here has usually proven sufficient for my needs: look for a synonym, mash multiple words together (though I draw the line at about four syllables), or change an ending to get the part of speech you need. Some dictionary entries have been created by tweaking Quenya words, so you could go with that if a Quenya dictionary has a word you need. It might not be "canon" Shadowlandian, but there aren't enough of us privy to the conlang to call you out for it if we happen upon your fic . Your Orcs could also simply use a loan-word from Common/English/Galactic Basic, and that'd give your narrative an opportunity to explain why they don't have the word in question.
Regarding "merchant", yeah, I'd go with "trader". Add the occupational "-al" suffix to whichever variant of "trade" you think sounds best, or "-og" to denote someone especially proficient in it. There's also the "-atar" suffix from Svartiska that serves a similar purpose, and a few occupational words in LOS pulled from that dialect use it. If you want to denote a traveling merchant, you might add "mûl" to your chosen "trade" word.
Regarding "scorpion", you could go with something like "baganazt" or "naztbaga" [poison sting]. A venomous stinger isn't exclusive to a scorpion, of course, but you could handwave that in your worldbuilding by saying it's the most common/threatening one around your orcs.
Regarding a diminuitive, "gaz" is "small", which I think would make it LOS's equivalent of "sta". I've also seen some fanfics use a small animal to fit this purpose (for example: calling someone "fîl-izub", my bird). Svartiska's "mikbork" is apparently a catch-all for "small animal", and I think "horngaz" would translate the same while being fully LOS-compatible.