This is NOT to cast any aspersions on OP's cooking abilities (and I'm also looking forward to reading an update), but I can easily see how this exact scenario could be an almost Ehell-approved attempt at not being rude.
Imagine this scenario: The guests love to see "the host and family" but have over the years talked about how "host"'s cooking is often very fatty (my beloved stepmother, for example, cannot cook a dish that doesn't have a sizable puddle of melted butter or lard on the bottom), or veggy, or carby, or whatever, to the point where they would normally cancel their attendance, if it were any other person but "host" whom they love and adore.
Ehell would debate at length what is and is not permissible depending on type of relationship, closeness, etc. I can easily see somebody saying "why not suggest getting the food catered to avoid the grease/confrontation, and offer to fetch it on the way offer, to sweeten the pot? If you offer to pay, though, it may cause too much commotion, "host" will start asking why which runs the risk of having to provide an explanation. Try to keep it low-key, like an afterthought.".
Again, I don't think that that is the case here but it is another perspective.
I get what you're saying and I think it's fine, as a guest, to offer to bring additional food. Dessert, for example, or maybe your own traditional sweet potato casserole, or some such.
But two things come into play there:
1. You have to accept "no" for an answer. The host has the right to say, "No thanks, I've got it covered."
2. I don't think you can extend this kind of offer if you mean the whole meal. Offer to bring a dish or two, fine. Offer to supply the whole meal, well, not so much.
In other words, if you're not comfortable accepting the host's hospitality, essentially as offered, for whatever reason, be it greasy food or you don't like their dog or their house is too cold or whatever, then don't accept it. Or figure out a way to deal with it, if, like you say, it's someone you really want to see. But once you do accept, I don't think it's polite to try to substantially change what they are offering.
I agree with everything you say (as usual
), but I am not certain it applies in the OP's situation.
I mean, as a general matter, yes, that's all correct.
But (and here comes my usual speech
), although it can be very tempting to turn to rules about "hosts" and "guests" and "invitations" to find clear guidance, those rules aren't always very useful, or, in my opinion, always applicable, in every situation.
A family holiday meal is a example. Sure, you can look at it as one family is the host and everyone else are guests, exactly the same as a dinner party. And I'm sure that in many families that is exactly how everyone sees it.
But in other families, it's more a matter of a family meal the same as if everyone all still lived in the same house, and this year, or every year, we're having it at Trudy's house because Dale is allergic to Carla's cat/Andrew doesn't have a big table/Lurlene lives way out in East Dorten, but that doesn't turn this into Trudy's private social event to decide everything -- how to set up the tables and which linens to use, sure, but not the "guest list," for example. (In that hypothetical family; again, I stress not every family looks at it this way, but many do.)
And the menu might well fall in the middle of that continuum.
If the OP thinks of it the first way, then it's certainly understandable that she'd feel intruded upon by any suggestions of changes, but the BIL may just be suggesting another way of doing it, not because he doesn't trust the OP's skills or to mooch a catered meal, but perhaps because he himself doesn't like cooking and thinks of it as a burdensome chore, so he's suggesting a way to make it easier for the OP.
I'm not saying that's what is going on -- the OP has told us this guy is annoying and moochy -- my point is just that there is more than one way to look at this and definitely more than one possible motive. The personalities as well as the context make a big difference. I wouldn't think a thing of it if my (very nice) BIL made suggestions about a holiday menu, or even suggested catering -- but I would find it truly bizarre if a dinner party guest did.