But even that does not increase the number of irregular verbs past two and a half (くる, する, ありえる). I don't know enough Japanese to say if one can invent forms like "swole" or "boxen" in it, so that they would remain recognizable.

Archaisms, slang, and alternate pronunciations definitely exist in Japanese. And dialects, of course.

You can invent a lot of things in written Japanese using textspeak and other alphabets.

Spoken Japanese you can't break the rules because everyone else instantly becomes unable to understand you. They barely use local accents/dialects in media because of this.