PigletsPal, I am in the same boat, sort of. We have FD booked 12/17-27...but DS has finals that are messing with our dates...the 18th is a far better option. Our ressies is in my name, and when I called WDW to inquire about the LATEST check in time to still keep promo valid...they said I have to be there on the 17th, (or 1-2 ish in the morning on the 18th, they understand about weather issues etc)and the rest of my party can check in on the 18th, HOWEVER, all dining credits room cost etc is as if the full party is already there..( I could go and eat on all their credits...lol...

) but that was fine with me, ...but another issue is the cancellation and paid in full policies since it is a package...with room only if anything comes up (we have 2 sets of parents in ill health) we prefer to have the room only/pay balance on arrival/6 day cancel policy. Ultimately, driving is more cost effective for us as well...which would be a very late arrival on the 18th IF weather cooperates from Illinois to Orlando 3rd week of December...

rental cars are insane at that time and we need one as we are seeing family as well on this trip...SOOOOOOOO, long reply, but yeah, at least the person the ressie is booked under has to get there the 17th. PLUS, we have checked in late like that before, 1 a.m., our room was in no mans land at POP...didn't ruin the trip, but not what we are used to.
I am VERY curious to see how the general public code goes...by way of FD or room discount...I imagine FD,as my thoery is when people have FD, they feel they have spare $ for souveneeer's etc., especially at the Holidays...I think merchandise is where WDW is hurting most right now, and when people are paid in full on a package, they may not watch their $ as tight as having just spent on a meal etc. Again, just speculating, but wouldn't surprise me.
Check out the Dolphin, you may be able to score a decent room rate there, it is our back up plan.