All these questions are about culinary-uses, which has always been a grey area on the site since its early days, and subject to case-by-case decisions. There is a discussion about the tag here on Meta.
Leftovers are one of the examples which are off-topic when it comes to culinary uses. In general, you can just eat leftovers. Repurposing them is not necessary, but if we start looking for this type of answer, we'll quickly end up with monotonous, overly broad lists, which is always a closing reason.
As for the three questions you mentioned, two of them were created and answered before the culinary discussion came up at all. They are from 2010, the first few days of the site, when Seasoned Advice policies and scope were still very much in flux and everybody was trying to find out what kind of question works and what doesn't. The Meta question on the culinary uses tag was asked in 2011, and the policy was only established after that point.
The newest question of the three was posted in 2013 by a new user and answered by users who are highly active, but came to the site long after these discussions have been settled. They probably didn't know the policy, so they didn't flag. It is not an especially popular question, so it seems that nobody else thought of flagging either.
In all three questions, you can see that they are exhibiting the problems we want to prevent. The new question, which is about crumb topping, has two answers: the one is to use it for its original purpose (i.e. make a traditionally topped dessert and top it with the topping), and the other one is to eat it as-is (after baking, which is a no-brainer in the case of raw dough). The goose question just lists general food groups which can be made with meat - soup, stock and curry. The sushi question is a bit more interesting, because of the safety angle. But once you take out the idea that it will be sufficiently safe for eating after a standard cooking process (which is common to all answers except a disputed one which insists that there is no safety problem), you also end up with a list of dishes.
So, all these questions produce an open list of ideas, each of them no better or worse than the others. This makes them closeable for both the "too broad" and "opinion based" categories. This covers both your cake question and the old questions you listed, which have simply been overlooked until now.
I will now handle the old questions too. I think that the food safety angle problem makes the sushi question worthwhile, so I will edit it into a non-leftovers question instead of closing. The can simply go.
The reason why you can't see your cake leftovers question is that it has been deleted. This is normal for closed questions, in general one shouldn't rely on a non-duplicate closed question to stay accessible after activity has ceased for more than a week or two (although in practice, we seldom enforce this, so there are lots of older questions floating around).
