The construction interesting to VERB implies that the person who is interested is the subject of VERB.
If I say "This video is interesting to watch", with no further qualification, I mean that when I watch the video I am interested.
If I say "This video is interesting for students to watch", I mean that when students watch the video they are interested.
Now: It is possible for VERB to be cast in the passive. For instance, if I enjoy being the center of attention I may say
It is interesting to be watched
meaning that when I am watched I am interested. And if I teach an entire class of self-centered students (which is not unlikely), I may say
It is interesting for students to be watched
meaning that when the students are watched they are interested.
But you cannot say This video is interesting to be watched, because that implies that when the video is watched the video is interested, which is nonsense—a video is inanimate, it has no feelings, it cannot be interested.
As to your final question: obviously you cannot speak of an interesting-to-be-watched video, or anything else. You might speak of an interested-to-be-watched student; that would be grammatically meaningful, but not at all idiomatic.
English really really dislikes putting adjectives in the 'attributive' position (before the noun) when they have following complements: it makes it difficult to parse the relationship between the adjective and its head, the noun it modifies. (That's why that sort of construction has to be hyphenated.) There are a few phrases which tolerate this sort of construction (easy-to-read, ready-to-wear, and the like), but I advise you to avoid it until you have a lot more experience with the language.
In "let's get the party started," the word "started" is used as an adjective. (It's a participle.) In "let's start the party," the word "start" is used as a verb.
In effect, since "'starting the party' gets the party started," the two are interchangeable. In some situations, phrasing the sentence so that "started" is the last word is preferable as it strongly emphasizes "started." On the other hand, the way "gets" is used in that form is not very formal.
Best Answer
Both questions are grammatically correct, but they have small differences in meaning.
In general, get + past participle is a way of expressing the passive voice. It is nearly identical in meaning to be + past participle.
Thus, "How did World War II get started?" has the same meaning as "How was World War II started?" This question asks about what forces or events caused World War II to begin.
"How did World War II start?" is similar in meaning, but the answers to this question could center around the events that occurred at the beginning of the war, as well as the events leading up to it. Either answer would probably be acceptable without further context.
As a side note, both of these questions are very, very broad, and the listener is likely to ask a follow-up question to narrow down the topic. A more focused question might be, "What were the major events that led to the start of World War II?"