All these past participles require have or -'ve

A) I was a little too fine tuned when I specified sibilant. Most if not all consonants will do the trick. This was common in Latin; nobody rails against such "errors" as Republic for res publica or irregular for inregular. It is also a feature of Arabic, the article al becomes as before words starting with s, am before words starting with m etc.

2) I'm speaking of the historical development of this process. This isn't something that happened overnight or even in one generation. The 've slowly got softer and softer till the youngsters learning the language didn't hear it at all so they didn't incorporate it into the grammar they developed from listening to their elders.