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.