I agree that many English speakers will understand the word, but consider the effect you are trying to make. With ergo, your listeners may think you're being a bit stuffy or pompous or are trying to show off your knowledge. "Therefore" is longer, but it's the common expression in this day and age and it won't make anyone feel uncomfortable.
As for "argal", I think that Shakespeare made up the variant to indicate that his gravediggers were so uneducated they didn't know how to pronounce the word correctly. Will had great fun playing with language and was not at all above writing in dialect and creating new words. In this case, he might even be recreating a dialect that he had heard on the London streets in the 1590s.
