I7-IV

If this chord progression is used, will there have been a modulation, unless the seventh of the I is preceded by a the diatonic seventh scale degree of the key?

There doesnt have to be, but it makes for a nice way to do it should you see fit. A modulation isnt automatic from any two chords. You need to see the section, or phrase as a whole.

This depends on the context. It may simply be a tonicization (I7 being secondary dominant or V7 of IV). It may be modulation if the following chords confirm IV as tonic. As it was said, you should consider the whole.

The definition of modulation differs from person to person. That can be considered a modulation, purely because if (with the right accents) you stop on chord IV and ask someone to sing 'do', they will mostly sing the root of chord IV.

And yes it is used frequently, but depending on the era there are different conventions, for example Classical Era music tends not to use it early on in the composition.