This is a case of catch 22 as well as what's supposed to happen vs what does happen. There are 2 rules. You must add the DP 2 days ahead and pay but everyone there during the week is supposed to be on the plan. It's not uncommon that these 2 rules are in conflict for those trying to follow the rules. Also, I've never heard of a
DVC front desk charging, or threatening a charge when one is adding a guest during a stay.
Obviously this opens the system up for abuses. My opinion is that it comes down to one's intent. If one purposefully omits guests they know will come then tries to add them later to avoid the DDP, that's not something I'm comfortable with. IF one truly doesn't know and is prepared to pay for the DDP if need be, one can leave it up to the FD. It's actually happened to us twice where my son (lives less than an hour away) came to visit and stayed unexpectedly. Both times we knew he might or might not but did not know for certain until he arrived.