We also sell finished pigs. Our alternative system actually makes us less money and is more profitable for the buyer - otherwise they would simply buy a finished pig - the advantage to us is a guaranteed buyer from a very early age.
If the customer chooses to periodically visit their pig, this need only take a half an hour visit once a month, considerably less time than it would take to feed and wonder at their pig every day, twice a day. The buyer most likely doesn't have the transport or time to take to slaughter, that is the point of the system, we take the pigs to slaughter.
With regard to benefiting both parties, we would have a guaranteed sale, the customer would have guaranteed high quality, rare breed pork, much cheaper than buying a whole finished pig - and certainly much cheaper than buying cuts every now and again. The whole enterprise depends on the customer being satisfied, every effort will be made to make sure it grows as expected. Without making light of the issue, if it doesn't grow as expected on our property, I can say with some confidence that it probably wouldn't grow as expected anywhere else.
We treat the pigs as our own, you cannot legislate against illness or death, these could happen to purchasers who buy their weaners conventionally and rear them themselves. So far we have never needed any veterinary input but if a customer's weaner needs treatment it would have to be paid for just as if the weaner was on the customer's own property. Likewise death, however unlikely, is an unfortunate part of life.
Customers would be welcome by prior arrangement so there wouldn't be a mass of visitors at the same time. Each buyer would have time dedicated to them and their pig to see their environment and have any questions answered.
With regard to your final point, it may well be easier just selling the pigs ready to go (as we also do) but this is something we have thought of to make the customer more part of the experience, their reward being inexpensive meat at the end of the day.