In my opinion, yes.
Yes.
I'm not going to provide any numbers, but I will venture to say that C will remain useful in the next few years after you learn it a few years from now. Actually, why not start learning it now?
Read and write C programs
C is a general purpose programming language.
If you are immortal, you can use C 20000 years from now. If you have a time machine and transport the right equipment with you, you can use C 20000 years in the past.
Yes.
But now that we have these questions out of the way...
You come across as having a "I'm going to learn C, and this is the only programming language I'll ever use to solve any programming problem I face, for the rest of my life" attitude. Unfortunately, this mindset will cause you to be a mediocre programmer. You should open your horizons and use the right tool for the job. You should even learn tools that may not be of immediate use to you, but which can give you better insight into the tools, such as C, that are of immediate use to you.