It means that the writer (and family) will be able to successfully (be okay) live independently, without assistance (on our own).

"the Old Sinner" here refers to the Devil. I don't know the specific term "guinea testimonial", but in this context, I think it means "endorsement", as in a statement endorsing someone's character. The sentence could be read loosely as "he could get us to sign an endorsement of the Devil's character".

It looks fine. You could make an improvement by saying that "the attached file contains the new metadata", but that is a minor point.

To be politically correct is to behave in a way that is meant to not offend anyone based on their membership in a larger group. An examples of behavior which would not be considered politically correct (or "politically incorrect") would be to tell racist jokes.

I believe the word you are looking for is intercessor.

Iris, while I can't give a firm answer without more context, my best guess is that the speaker means to say that she is taking two science classes.

I believe that what you're looking for is a difference is usage. For example, a person, when asked if they want to participate in an activity, might respond with either "sign me up!" or "count me in!", and both expressions would be more or less equivalent. They also would both be informal expressions. The more general activity of "signing someone up for something", though, would indicate a more formal activity of enrolling (such as for a class or other formal group activity), whereas "counting someone in form something" would generally be for a more informal activity, such as going on a hike or out to a club with friends.