Learn English – Is it common for English people to say “crap” instead of “crab”

This phenomenon happens all the time. What we call "language" is nothing more than the words that people are actually using. When enough people use a word or pronunciation or spelling for a long enough time, it becomes part of the language. How long this takes is hugely variable, and depends largely on how useful people find the new construction to be. Thus, new tech jargon is assimilated very quickly. Other things (like "gaol" becoming "jail") take hundreds of years.

"Supposably" is very far from the point of becoming standard English. The reason that "supposable" sounds so wrong is that you expect the "-able" adjectival ending to be used with transitive verbs--the object of the transitive verb becomes the thing described by the "-able" adjective. Typical usage of "suppose" isn't really transitive, so "supposable" sounds wrong. The first step toward "supposable" becoming accepted is for transitive usage of "suppose" to become common.

The general rule of thumb is to read it as you would any other word, i.e., follow normal rules of pronunciation. It'll work most of the time, and when you're off, you'll be told the correct, "unorthodox" pronunciation.

I can't speak for all Americans; however, in my elementary school we did read texts aloud, but rarely. And I don't remember specifically being taught how to pronounce unfamiliar proper nouns.

I say don't worry about it too much--if it's unfamiliar to you, it's likely to be unfamiliar to many others. In this case, no one will fault you for not knowing how to pronounce the word; as you said, it can't be looked up in a dictionary.