This is a great question to ask. I'll start by saying that whether something was voted as on or off-topic in Area 51 should have little or no bearing on this. This should be decided based on the discussion that will occur in this thread.
I'll address each of the topics in your bullet points:
Wine pairing, temperatures and preparation (i.e. decanting)
I consider this on topic. Does it broaden the scope of the site? Yes, it does but I think the added value is worth it. I think that a scope of "food" where food is defined as an edible solid, and a scope of "cooking" defined as applying heat to food until it is done, is simply too narrow and restrictive. World renowned culinary institutes who, by definition, churn out expert chefs all require classes in wine pairing. Granted they aren't at the level of a sommelier, but they typically have beyond a layman's level knowledge. If this site happens to attract expert sommelier's, I only see that as a benefit to the community. Many dishes can only be fully expressed with the proper pairing as recommended by the chef. I'll reuse this point for the other topics, but I strongly feel that "food and cooking" is strongly coupled to beverages, and includes full meal experience.
This can become a gray area. However in the context of food presentation I think this is very much on-topic. Again, I'll return to the expert chef example. An expert chef is expected to have food presentation skills. Presentation is essential to a great meal experience. Assuming we do attract experts chefs, they will have this knowledge, and it will provide a benefit to the community. As an example, but not a reason (there's a difference), look at any televised cooking competition or even the penultimate Bocuse d'Or, presentation is very much a part of the judgement of dishes.
However, I'd say we would draw the line at food presentation. I wouldn't consider floral arrangements, tablecloths, or other environmental accessories as appropriate for this site.
Dinnerware, table-setting, etc.
Eh. I don't feel as strongly about this as I have the previous two. This can sort of blend with presentation. I'd say that table-setting is too far out there. However, choice of plate can be tied into presentation. An example is one of my favorite Chicago restaurants Bonsoiree, some of their dishes use unconventional dishes for presentation that really accent and add to the enjoyment of the meal.
Cocktail mixing (IMO)
I also don't feel that strongly about this. Of all the bullet points this is the most likely to be able to sustain it's own site. However, please note that I'm referring specifically to the mixing of cocktails. Cocktail should not become a dirty word here. If cocktail mixing itself were deemed off-topic, I would still consider as valid:
- Can I serve xxx with a cocktail?
- What cocktail would go with xxx?
Serving equipment (from chafing dishes to pizza cutters)
Eh. Really broad topic in itself. I see a pizza cutter as a tool like a knife more so than a piece of serving equipment. In that aspect, I'd say it is ok. However presentational dishes? Eh, some might see this as contradictory to my answer on dinnerware above, but I'll point out again that I don't feel that strongly about these. I think that serving dishes are a little to Home & Gardeny a topic for this site.
Course planning (menus)
I absolutely think this is on topic. This falls under my belief that food and cooking should cover more than just the time in the pan. Cooking is much more than this. Pairing an entree with appropriate appetizers, side dishes, and desserts is knowledge an expert chef will have. They are taught this, and are expected to know it. We can only benefit from this knowledge.
Basically, I think that having the scope of this site defined too narrowly is more of a detriment and risk to the future success of this site than a reasonably broad scope. If we look at the amount of knowledge and the breadth of knowledge that an expert chef is going to have, we should be very hesitant to exclude a subset of that knowledge from this community. This type of exclusion does have the potential to exclude an expert chef.
Another thing to keep in mind is that cooking is as much art as it is science, if not more so. I've seen it suggested by others, and I am inclined to agree, that we need to accept a greater degree of subjectivity and creativity in the content of this site than we do in StackOverflow. Programming is logical, there is often only one answer, or just different ways of giving the same answer. Approaching Food and Cooking as if we're trying to create some Turing complete community for cooking is doing the potential this community has for greatness harm.
I didn't initially address your "Will it make us more or less unique than other sites?" question. I'll do that briefly here.
I think that our uniqueness from other sites is guaranteed. The whole StackOverflow concept is unlike anything that has been done before. Yea, there have been wikis, forums, expert exchanges, etc. in the past, but noone has ever rolled them into one and actually made something amazingly useful out of it. The format of this site alone is going to make us unlike anything out there. Hell, these discussions we have make us unlike anything out there. We shouldn't get hung up on trying to narrowly define our scope as a way to make ourselves unique, we already are. (To those who prefer to forgo logic when reading, this in no way implies that we should have no regard for scope.)
I also want to address this statement of yours: "Nevertheless, here we are, 1 day away from the public beta, so the question we desperately need to answer definitively ...". I don't think we do. I don't think there is a desperation or that we need to have a definitive answer. Not to mention that we likely won't. We are a small minority of the future vast community this site will become. Yes, I've read the 7 Essential Meta Questions. I'm aware that this is the phase where we sculpt and determine the future course of this site. That doesn't mean we have a deadline to decide every important and far reaching decision. That link even states that it took a year to iron out what topics were considered on and off topic for the FAQ. The guidelines there encourage us to "Talk about these issues in meta, early and often." This is what we are doing. Take note of the often. This does not equate to "Decide these issues early." That's a very different thing.
If anything I think one of the conclusions we need to take away from private beta, and into public beta is that we won't have a definitive answer for this. Because we are a community and we make the rules here, it's perfectly acceptable to recognize that making rules for this Food & Cooking community is going to be significantly more difficult and intricate than for StackOverflow or ServerFault. We should acknowledge this gray area, embrace it, and use the blade of objectivity to gently shave and sculpt our site not hack and slash.