"I like my mother so much."
Translation:Eu gosto tanto da minha mãe.
Native English speakers are not authorities on Portuguese either so I'm inclined to disregard your definitions, especially when a native Portuguese speaker has said that using "muito" should be fine.
And for what it's worth, native English speakers are not authorities on English either.
My source was an educated native Brazilian with linguistic training, who, in case you missed the subtlety, assured me that (like Spanish of which i am a native speaker) muito and tanto can be interchangeable in such constructions. The general native speaker of a language can produce with remarkable exactness correct sentences in the dialect s/he knows. There are preferred dialects, of course, but just as few of us can explain how we know how to produce fingernails and hair, few uneducated speakers can tell HOW the language works, what alternatives are, how the grammar works and what the dialectical preferences are. That's part of what education does. Linguistics does it more and better. So i trust my training and o meu amigo more than your opinion.
In Spanish (which has many Iberic similarities but is different) this placement sounds better and the traditional syntactic rule of thumb is that "the shorter the qualifier, the closer it goes to the head word" -- so tanto (an adv.) goes closer than da minha mae (a prepositional phrase), unless you want to make it emphatic -- the result of putting things in untypical order. I guess there is a similar rule in Portuguese.
Its more of a "street rule" as the way it is setup in duo is what you tend to hear in normal conversation when someone is talking about something or someone that they like so much; I think i heard it at the end once in a while, but the way it is far more common (at least in Brazil, dunno about Portugal)