Are honorifics used for dead people? For example, would チンギス・カン have something after the name (presumably not just a mere さん!) when you're not referring to the dish?
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Since honorifics generally apply when you are interacting directly with that person, they usually aren't used for people that have died (Maybe spiritual mediums do something different because they are supposedly 'interacting directly' with the deceased, but I don't know).
Think of great people from Japanese history: 織田信長（おだのぶなが） 徳川家康（とくがわいえやす） 聖徳太子（しょうとくたいし）, all of them are referred to just by their name. Deceased former prime ministers are simply referred to by their name. Deceased Emperors are called by their reign (i.e. what we call Emperor Hirohito is just 昭和天皇（しょうわてんのう） the Showa Emperor in Japanese), while the current living Emperor Akihito is referred to in Japanese by 天皇陛下（てんのうへいか） or His Majesty the Emperor.
Going closer to home, people refer to their deceased relatives the same way they did while they were alive. Buddhist beliefs give the deceased a new name for the afterlife, but I have never heard of this new name actually being used to refer to the deceased.
I would say yes. For example, if you were talking about a deceased teacher, it would be perfectly natural to say (for instance) 先生が亡くなられた後..., which uses an honorific. The honorific 先生 would also be used.
If a person personally knew and had a relationship of some kind with 岸信介, I suggest that even now such a person would talk of 岸さん and use appropriate honorifics. History books, on the other hand, would refer to him as 岸.
Genghis Khan and other long-dead historical personages don't count, unless you happen to belong to a group that sees itself as directly (religiously?) connected to Genghis Khan. It's the relationship that's important, not whether they are alive or not.
You could use the prefix
例： 故斉藤氏 → The late Mr. Saito
BTW, thanks for making me hungry for ジンギスカン （ブツブツ）...