No, "witnesses" in the passages that speak specifically about using their "mouths, words, and testimonies." cannot possibly mean "objects". Do "objects" have literal mouths? Can objects literally speak words? No.
I see what you are saying that evidence is used in criminal cases and evidence is used to prove or disprove something. But if you were the prosecution in a court of law and you said, "I'd like to call my next witness, this knife." You would be looked at like you were mad. What you would say is, "I'd like to point to the evidence, this knife." right?
I think I understand what you're saying. Just in the context of the passages you quoted, "2 or 3 witnesses" can only be referring to 2 or 3 actual people.
If you see something, it's real. You can't see something that isn't real. So no, observation alone is enough to prove to yourself whether or not something is real, at least it is to me. I've never seen something that wasn't real.
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“But if a man finds a betrothed young woman in the countryside, and the man forces her and lies with her, then only the man who lay with her shall die.But you shall do nothing to the young woman; there is in the young woman no sin deserving of death, for just as when a man rises against his neighbor and kills him, even so is this matter.For he found her in the countryside, and the betrothed young woman cried out, but there was no one to save her. - Deuteronomy 22:25-27
http://www.biblegateway.com/passage/?search=Deuteronomy22:25-27&version=NKJV
According to your claim, that "mouths of two or three witnesses" means "eyewitnesses", as in "persons". How then could you even accuse the rapist in the above passage? There was no one other than the rapist and the woman, so according to you, that rapist should be able to walk away, scot free, because there was no one else around.
But that's not what God says to do, which is to execute the rapist, and spare the woman. How can they do what God says without "eyewitnesses"?
In order to have a conviction, then there has to be at least two witnesses:
“One witness shall not rise against a man concerning any iniquity or any sin that he commits; by the mouth of two or three witnesses the matter shall be established.If a false witness rises against any man to testify against him of wrongdoing,then both men in the controversy shall stand before the Lord, before the priests and the judges who serve in those days.And the judges shall make careful inquiry, and indeed, if the witness is a false witness, who has testified falsely against his brother,then you shall do to him as he thought to have done to his brother; so you shall put away the evil from among you.And those who remain shall hear and fear, and hereafter they shall not again commit such evil among you.Your eye shall not pity: life shall be for life, eye for eye, tooth for tooth, hand for hand, foot for foot. - Deuteronomy 19:15-21
http://www.biblegateway.com/passage/?search=Deuteronomy19:15-21&version=NKJV
If there is no conviction, there can be no execution, because God does not authorize executions without a trial.
Do you see the problem yet?