Welcome to Coin Community, silvermaniac. You post some interesting and valuable questions, and here are a few thoughts regarding them:
The dies used to strike coins have a lifetime, and over their striking career are capable of showing increasing wear. In addition, minute differences in the quality, composition and dimensions of the planchet (the original coin blank) can have a bearing on the look of the final coin. I'm thinking the Halfcrown shows evidence of this - the striations towards the rim are caused by the striking process, and might be more prominent if the die is worn and nearing the end of its' life. So, also, could the planchet contribute to such an appearance.
A "fresh" coin struck from a worn die could show less detail than a circulated coin struck from a brand-new die, as a result.
Now, regarding the 500 Reis. Occasionally, during the minting process a planchet fails to be fed into the press and the dies come into contact. It's possible when this happens for details of one die to be impressed upon the other, sometimes strongly enough for those details to be passed on to the next few coins fed. This is called clashing, and I believe it's what you're seeing here. The "curl" coming off Pedro II's forehead is a detail from the reverse, transferred to the obverse in a clash.
The 20 Kopecks appears to be the result of a very poor die polishing process; polishing impressed lines onto the die strongly enough to be transferred to coins struck by it. This is not uncommon; look closely enough at almost any USA Cent and you'll see what I mean.
You have been lucky enough to have seen few coins whose dies received such indifferent treatment, but they are not uncommon.
Not all minting operations involved quality control as strict as others, Especially during that time of great tribulation in Russia, I could imagine quality control slipping to the point where off-weight planchets were allowed into the system; it could also (especially if the coins are underweight) be the result of corruption in the system - organized "skimming" of the metal used in the planchet.
In summary, there are believable explanations for all these which would indicate all of the coins are real, just having been victimized by various problems inherent in the striking process. Please forgive me if I have been too simplistic in my explanations; I'm also addressing those who read this thread without ever posting in it, who have greater or lesser knowledge of minting operations.