After thinking about this, I think the key is not letting the gray area of when the Alchemist can alchemise get too big. Yes he can use this ability more or less at will, but you cant do two things at once. If your shopping, you cant alchemise while you are buying things. But you could immediately before and/or immediately after. Once youve declared your alchimise, you cease to be shopping and cant buy something becuase you flipped it into more gold. That should prevent this perceived exploit of the alchemist.
To the original question, I thought it unbalanced it to have someone not in the city be able to suck someone back into the city from anywhere so I ruled that such things only affect the targetted character when in The City and that if the outside character landed on the City corner or returned to the city, he would immediately be affected (thrown in jail).