Does a dominant seventh chord always have to function as a secondary dominant/cadence? In "I Wanna Hold Your Hand" they play a B7-C and a B7-G instead of resolving to an E. Does this just work as a bluesy kind of sound?
Are there cadences that work well besides the perfect cadence and plagal cadence?
Does a seventh chord always have to function as a secondary dominant/cadence?
In traditional western harmony dominant seventh chords are reserved mostly for dominants and secondary dominants while fully diminished seventh chords have a similarly dominant function. However in romantic-and-onward harmony seventh chords (as well as 9th, 11th, etc.) are often used coloristically, meaning they are used to color the root triad or add harmonic richness and ambiguity.
See, for example, the amazing harmonic colorings Prokofiev does here in the second movement of his Sixth Piano Sonata:
(Cued recording here)
Here he takes a melody and harmonic progression which simply goes back and forth between I and V and makes all kinds of alterations to it, adding and modifying pitches left and right which don't have much intuitive voice leading to speak of (parallel fifths abound). The object of his game here is to take simple material and make unexpected and surprising transformations on it. The underlying harmonic structure remains the same despite the various sevenths and non-chord tones added everywhere.
There are as many cadences as there are melodies - just try things out and see what works!
Does a seventh chord always have to function as a secondary dominant/cadence?
No; this isn't really the case in modern music (including but not limited to pop, rock, jazz etc). I'll take Jazz as a reference point, because jazzists make every chord a 7th chord. You can add another third in any triad (chord), and you would have a 7th chord. This won't make it necessarily a (secondary) dominant. It will add color to the chord.
Are there cadences that work well besides the perfect cadence and plagal cadence?
There are the deceptive (or Interrupted ) cadences. To quote Wikipedia:
V to vi. The most important irregular resolution,most commonly V7–vi (or V7–♭VI) in major or V7–VI in minor.
This is really really common in popular music (and common practice period as well). The first cadence you mentioned, B7-C falls in this category.
Now, jazzists have more deceptive cadences. Pretty much any chord that sounds right after a dominant, fits.
There is (at least) one chord in Common Practice Period harmony that has the same interval-pattern as a dominant seventh that behaves differently, the German Sixth. It's normally written differently (Ab-C-Eb-F# in C rather than Ab-C-Eb-Gb) but it's possible to play enharmonic games with the chord.
Whether a chord is a dominant seventh is more accurately determined by its resolution. The above chord (Ab-C-Eb-F#) normally resolves to a C64 (major or minor) followed by a G chord when acting like a German Sixth but to a Db chord when acting like dominant seventh. In the first case, the chord has a pre-dominant or sub-dominant function, in the second, a dominant function. As a sub-dominant, the augmented sixth resolves outward to a duplicated dominant note (not necessarily the chord) and as a dominant, the root moves down a fifth like any other dominant.
