I looked at the files of some other texture packs, and apparently both the Guide and Travelling Merchant have separate arm sprites, Extra_52 and Extra_53 respectively. News to me, but I didn't know if you'd already figured it out.
Anyway, with that knowledge, I just put together a little fix. I'll put the files here if anyone wants to use them, unless you've already done them/want to do them manually
(I haven't actually tested it with the Travelling Merchant though, since he's hard to get a hold of)
View attachment 315046