Learn English – What’s the meaning of ‘boil out of ~’

Though the complex preposition of off of is widely used, one may argue that it's redundant.

Here is the Oxford Dictionaries' reference:

Off of is often used in place of the preposition off in contexts such as she picked it up off of the floor (compared with she picked it up off the floor ). Although off of is recorded from the 16th century (it was used by Shakespeare) and is logically parallel to the standard out of, it is regarded as incorrect in standard modern English.

Now, about the point

someone got a point - someone got an idea to think upon

In this case, "the whole ground" is a metaphor meaning a very wide range. So his "gifts, and instincts, and feelings, and interests" cover a lot of topics. To put it another way, he is interested in a lot of different things, and feels a lot of different things, and so on.

Mark Twain is a delightful author, but he is best known for how he captured the patterns of speech of the poor and uneducated in the late 1800s. I highly recommend his works, but they are probably not the best source if you are trying to learn English. His idioms, sentence structure and often his spelling are archaic now, and were often considered wrong even at that time by educated people.