To answer the "Where's the Splenda" condition, it's a question of availability. The manufacturer apparently underestimated the amount of demand for Splenda, and we're actually in a shortage situation right now until a new facility is online.

The company I work for has contracted for a certain amount of Splenda per year at $XXX/lb (yes, it is a 3-digit price per lb - but it's >600x the sweetness of sugar). Any new products we come out with above the quota have to pay roughly quadruple the price. And, if they can put that kind of pinch on the largest US-based packaged food company, imagine what they can do to smaller food manufacturers.

Of course, they may not be rushing the new facility too much if they're still selling the product for >$2000/lb to non-contract customers.

On the safety issue, I'm well aware of what it takes to get a food ingredient certified onto the GRAS list (Generally Recognized As Safe). It's not trivial. Basically, what's involved are clinical studies to define what the upper limit of ingestion for a person should be per day to confirm no ill effects. The upper limit is generally very conservatively stated (ie the daily intake of Splenda might be based on the edge case of a person who drinks in excess of a gallon of diet soda per day).

The whole "exchanging of atoms" thing is a red herring at best if you're talking about swapping hydroxyl (-OH) groups for chlorine. For example:

Switch a -H to a -OH, and you've gone from a flammable gas (ethane) to a flammable liquid with fun side effects (ethanol).

Switch from that -H to a -CO-CH3, and you've got a synthetic banana flavor (ethyl acetate). Extend the chain a bit, and you're at a pear-like flavor.

Change the orientation of -H and -OH groups in a sugar, and you've gone from sucrose to lactose, and you've caused gastric distress in those who lack a lactase enzyme.