OPEN tickets refer to dates/times of travel. If the itinerary was open, how could you determine the number of continents flown (and the price)? The change fee is reasonable considering what the fee on a simple return ticket costs, usually $100. If you have many changes, do them all at once as it will only cost you the fee of $75.

Yes, you can route Europe-LAX-GIG and then later return to N.America. It is done all of the time with one being a transit without stopover. The best example is LHR-LAX-SCL which during the N.American winter time is a stopover of 23 hours. If you pass thru any other US city, the arrival is in time to make a same day connection to S. America.