When speaking, some of these naturally occur, especially when people speak too quickly. But not all of them are used or common. YES: Shouldn't've, wouldn't've, mustn't've, mightn't've, couldn't've, d'y'all, I'd've, she'd've, he'd've, he'sn't, it'd've NO: you've'n't, I'ven't.

I hope that helps!

It can definitely happen in spoken english. Also, the sounds often get abbreviated. EX: Should've, Would've, etc. become Shoulda, Woulda. There's even an expression "shoulda, woulda coulda," that basically means someone didn't take advantage of opportunities. When you add the double (negative) you often hear something like: Shouldna, Wouldna. EX: "He really shouldna done that." These are almost never written, unless someone is writing dialogue.

Hi! Hope Melody was able to answer your question. Just remember that double contractions are never used in formal English, and I have personally never seen them represented in text speak. It would be helpful if you look into syllable stress in American English to better understand which syllables get reduced.

These other answers are very good, but I thought I'd add what English speakers often say instead of "you'ven't" and "I'ven't". We often just get rid of the h sound in "haven't". For example, "you aven't" and "I aven't". We also do this with all the other pronouns. Great questions and good luck on your English journey!