ATTENTION Sunny Squire & others who plan the above!
Hoo tried this...and it didn't work!
After completing 49 trades, she paid the 5 gems to skip the last trade on the page in the goal book while #50 was out ... It came back with ONLY a trade ticket (no mitt!).
So, in order to keep a mitt, we'll have to do like Nat did, and skip the "Give 50 mitts" after the trading is completed (which thankfully, also only costs 5 gems).
Then we'll not only have 1 mitt in our Hoarder inventory, but FIFTY! Lol
Hope this news isn't too late for those others who were gonna try the first route & I managed to save you 5 gems.