I don't think so.
when adding a prefix, the spelling of the root word doesn't change
Yes it can.
I don't think so.
The world as we know it comes to an end. Just kidding. Many words, like "infection", have both a prefix and a suffix. You'll still need a root word. Some words, like "rely", look like they're just a prefix and a suffix, but they're not.
An example of a base word that can be modified by adding a prefix or suffix to create different words is "act." By adding the prefix "re-" you get "react," and by adding the suffix "-ion" you get "action."
raging
No, comforting is a suffix. Take of the -ing and you have comfort.
No, "knowledgeable" is not a prefix. It is an adjective formed by adding the suffix "-able" to the noun "knowledge."
You can make "flexible" into a noun by adding the suffix "-ity" to create "flexibility."
Sometimes yes, the spelling of the original word may change when adding a suffix, particularly if the word ends in a silent 'e' or a consonant preceded by a single vowel. In such cases, the final 'e' may be dropped or the final consonant may be doubled before adding the suffix.
Yes, these are called "prefix-bound" or "suffix-bound" words. They are words that alter their pronunciation when combined with a prefix or suffix.
No, "painless" is not a prefix. It is a standalone word formed by adding the suffix "-less" to the root word "pain."