There are ample passages from Baha'u'llah as to Abdul'baha.
Note the bold, this knowledge Abdul'baha said is from Baha'u'llah as the appointed Interpreter of the Message of Baha’u’llah.
“O Thou Who art the apple of Mine eye!” Bahá’u’lláh, in His own handwriting, thus addresses ‘Abdu’l-Bahá, “My glory, the ocean of My loving-kindness, the sun of My bounty, the heaven of My mercy rest upon Thee. We pray God to illumine the world through Thy knowledge and wisdom, to ordain for Thee that which will gladden Thine heart and impart consolation to Thine eyes.” “The glory of God rest upon Thee,” He writes in another Tablet, “and upon whosoever serveth Thee and circleth around Thee. Woe, great woe, betide him that opposeth and injureth Thee. Well is it with him that sweareth fealty to Thee; the fire of hell torment him who is Thine enemy.” “We have made Thee a shelter for all mankind,” He, in yet another Tablet, affirms, “a shield unto all who are in heaven and on earth, a stronghold for whosoever hath believed in God, the Incomparable, the All-Knowing. God grant that through Thee He may protect them, may enrich and sustain them, that He may inspire Thee with that which shall be a wellspring of wealth unto all created things, an ocean of bounty unto all men, and the dayspring of mercy unto all peoples.”
Baha'u'llah allowed Abdul'baha to offer replies on behalf of Baha'u'llah.
All this is available and is self explanatory, there is no negating the Station of Abdul'baha, a Station that was given in Baha'u'llah in His own handwriting and was a Sealed Testament to all that choose the name Baha'i.
To negate that station, is to negate the name Baha'i.
Regards Tony