You do not properly understand why Mormon baptisms are invalid so you are drawing an incorrect conclusion about Arian baptisms. Perhaps it would help if you reread the passages that I have cited. Ladislaus is giving good explanations too.
Please stop being presumptuous. There is nothing in what I have written that indicates a misunderstanding unless you take me out of context. I agree precisely with all the quoted passages, and I maintain that ARIAN BAPTISMS are invalid just as are MORMON BAPTISMS because both, in denying the Trinity, mean by "Father, Son, and Holy Spirit" something entirely different from what the Church means by them, to the point that their intention in administering and receiving Mormon baptisms, in the context of their baptismal rites, cannot be the to do what the Catholic Church does, i.e., he intends to do what the LDS church does and that is to baptise someone into the LDS church and its three strange beigns of Mormon theology and only Mormons know what else, and this intention is blatantly and publically manifest. THis is totally different to the case of a Mormon performing a Catholic baptism.
The document I referenced, which clealry states that there is a deficiency in intention of the celebrating minister of a Mormon baptisms due to their understanding of God:
III. The Intention of the Celebrating Minister.Such doctrinal diversity, regarding the very notion of God, prevents the minister of the Church of Jesus Christ of Latter-day Saints from having the intention of doing what the Catholic Church does when she confers Baptism, that is, doing what Christ willed her to do when he instituted and mandated the sacrament of Baptism ...... The Mormon minister, who must necessarily be the "priest" (cf. D&C 20:38-58.107:13.14.20), therefore radically formed in their own doctrine, cannot have any other intention than that of doing what the Church of Jesus Christ of Latter-day Saints does, which is quite different in respect to what the Catholic Church intends to do when it baptizes, that is, the conferral of the sacrament of Baptism instituted by Christ, which means participation in his death and resurrection (cf. Rom 6,3-11; Col 2,12-13).
If there is error here it was in my saying that they also have valid form, when it's just the objective formula but not the substance of the words (as I said, one may as well baptise in the name of Hindu deities). But this is semantics: if we look at form instead of intention, the same argument applies again to both Mormons and Arians. Arians understood somehting radically different by the Trinitarian formula.
An Arian performing a Catholic baptisms, fine, but an Arian performing an Arian baptism? How is that ontologically different to a Mormon baptism?For the Arians do not baptize into Father and Son, but into Creator and creature, and into Maker and work. And as a creature is other than the Son, so the Baptism, which is supposed to be given by them, is other than the truth, though they pretend to name the Name of the Father and the Son, because of the words of Scripture, For not he who simply says, 'O Lord,' gives Baptism; but he who with the Name has also the right faith. On this account therefore our Saviour also did not simply command to baptize, but first says, 'Teach;' then thus: 'Baptize into the Name of Father, and Son, and Holy Ghost;' that the right faith might follow upon learning, and together with faith might come the consecration of Baptism. St. Athanasius, Against the Arians, 2:18:42.
Here Athanaius plainly says that though the Arians (after Nicea) use the name sof Father and Son, they really baptise into the "Creator and creature" because of their false understanding of the Trinity.
Also, Mormons aren't heretics. One has to be a validly baptised Christian to be a heretic and a believer in the Trinity and Incarnation to be Christian. I'm not sure I'd call Arians, at least post-Nicea, heretics either.