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There are people who, no matter how much they claim to hate losing control or having chaos descend, actually revel in the chaos, as it makes them the center (frazzled, put-upon, stressed out) of the activities. Just the fact that the LW is pulling her hair out over the "crazy" relatives points to her unwillingness to take control and act like a grownup. She wants to be rescued instead of making hard decisions herself (e.g., she could have arranged the christening 6 months earlier, as originally planned). As someone else pointed out, she's signed as "It's MY Party (emphasis added)."
She somehow feels that every detail (including the needs of disabled relatives) is up to her, and only her, to resolve. This is egotistical nonsense. If the relatives can travel, they should have an inkling as to how to get about locally, and can enlist the assistance of relatives other than the LW.
There were good suggestions by others as to how to prevent people from camping at one's abode. They should be taken seriously.
The thing is, if the LW truly took control and refused to be blitzed by problems real and imagined, then she'd have no rationale to cry "poor me, poor me," which she's (I'm guessing) bound to do on and about the day of the ceremony. Family events are rarely perfect, but making oneself helpless is just fodder for craving attention and an excuse for things not going well. She needs to grow up, at least for the sake of her baby.
BTW, I don't get all the negative reading into her comment about "contacting the church." Wouldn't the church have to be contacted in order to arrange the date, as one would a wedding or any other church event?