Before 4 OMENS: I believe this is correct, if they become UNWELL, they cannot receive him. This does have repercussions, later. Taking it a step further, if you FED IT and then never opened the carrier, you would still lose out at C4.
Before 5 OMENS: That sounds correct also, but I still weirdly want to put some kind of back door there to get the BLACK PLANTER after-the-fact. I don't believe missing it is "fatal", but would prevent one or two possibilities of the 666. Edit: I've made an alternate route for C5+ to get the planter, if they missed the earlier opportunity.
XMAS TREE: I think you're able to get it whenever. I don't believe it's tied to anything but exploring that place.
GUESTBOOK: Correct.