There were the "three witnesses" who saw both the Angel and the Plates.
Very well, I accept that there were individuals who claimed to be first-hand witnesses. However, this is only the first test which YHVH Elohim taught us to identify false prophets.
Secondly, did this following prophecy come to pass? Was a LDS city & temple built in Missouri when Joseph Smith was alive (D&C 84:1-5), and was it dedicated by Joseph Smith's hand?
1 A revelation of Jesus Christ unto his servant Joseph Smith, Jun., and six elders, as they united their hearts and lifted their voices on high. 2 Yea, the word of the Lord concerning his church, established in the last days for the restoration of his people, as he has spoken by the mouth of his prophets, and for the gathering of his saints to stand upon Mount Zion, which shall be the city of New Jerusalem. 3 Which city shall be built, beginning at the temple lot, which is appointed by the finger of the Lord, in the western boundaries of the State of Missouri, and dedicated by the hand of Joseph Smith, Jun., and others with whom the Lord was well pleased. 4 Verily this is the word of the Lord, that the city New Jerusalem shall be built by the gathering of the saints, beginning at this place, even the place of the temple, which temple shall be reared in this generation. 5 For verily this generation shall not all pass away until an house shall be built unto the Lord, and a cloud shall rest upon it, which cloud shall be even the glory of the Lord, which shall fill the house.
Does the above testimony fulfill the requirements of YHVH Elohim as He stated in Deu 18:22?
אשר ידבר הנביא בשם יהוה ולא־יהיה הדבר ולא יבוא הוא הדבר אשר לא־דברו יהוה בזדון דברו הנביא לא תגור ממנו׃ ס
When a prophet speaketh in the Name of YHVH, if the thing follow not, nor come to pass, that is the thing which YHVH hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him.
LDS don't rest our authority on "scripture", our authority comes directly from God by Revelation and the visitation of Angels, including the Father and Son themselves.
Scripture supports the idea that all new doctrine must conform to the authority of the Torah: (Isa 8:20)
לתורה ולתעודה אם־לא יאמרו כדבר הזה אשר אין־לו שחר׃
To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.