JOEL - Here are the scriptures in question:

D&C 42:15
"And all this ye shall observe to do as I have commanded concerning your teaching, until the fulness of my scriptures is given."

D&C 42:56-61
56. Thou shalt ask, and my scriptures shall be given as I have appointed, and they shall be preserved in safety;
57. And it is expedient that thou shouldst hold thy peace concerning them, and not teach them until ye have received them in full.
58. And I give unto you a commandment that then ye shall teach them unto all men; for they shall be taught unto all nations, kindreds, tongues and people.
59. Thou shalt take the things which thou hast received, which have been given unto thee in my scriptures for a law, to be my law to govern my church;
60. And he that doeth according to these things shall be saved, and he that doeth them not shall be damned if he so continue.
61. If thou shalt ask, thou shalt receive revelation upon revelation, knowledge upon knowledge, that thou mayest know the mysteries and peaceable things that which bringeth joy, that which bringeth life eternal.

D&C 35:20
20 And a commandment I give unto thee that thou shalt write for him; and the scriptures shall be given, even as they are in mine own bosom, to the salvation of mine own elect;

I don't know of any church authority who said that the JST is not scripture "on the same plain" as the 4 standard works. The JST is the Bible so it is scripture. The Church only chose the King James Bible as the official version because it is the most accepted version in the English-speaking parts of the world today. In fact Joseph Smith commented that he preferred some aspects of the Martin Luther translation over the King James(HC 6:307, 364).
The Lord Himself told us to "hold thy peace concerning them, and not teach them until ye have received them in full" (D&C 42:57). Therefore, since we have not yet received it in full we do not use the entire JST as the official version in our cannon of scriptures, even though it is scripture.
That part which we know is complete and correct is included in our LDS King James version either as foot notes or at the end of the Bible. If it were not considered scripture it wouldn't be there. This fulfills the command of God that they shall be "taught unto all nations, kindreds, tongues and people."(D&C 42:58)

MICHAEL - Lastly, how come modern LDS Prophets have failed to 'complete' the JST when D&C 42:61 says they can?"

JOEL - As I have said elsewhere the JST was very close to complete already, but because of the distraction of persecution, the demands of Church business, and the lack of financial means, Joseph Smith was not able to release a finalized manuscript ready for publication.
The modern prophets have not "failed" to complete it. In order to fail at something you need to first be given the task to do it. This was a task that was given only to Joseph Smith by God.
Speaking directly to Joseph Smith God said:

"Wherefore, I give unto you that ye may now translate it, that ye may be prepared for the things to come." (D&C 45:60-61.)

D&C 35:20 was given directly to Sydney Rigdon commanding him to write for Joseph Smith(See verse 3).
God has not yet instructed anyone else to "complete" it, so they are not concerned with it now; perhaps sometime in the future. Also, because the JST is published and copyrighted by the Reorganized Church(Community of Christ), the law of the land forbids the LDS church from publishing its own "completed" version of it.