I read somewhere : Molto=Much,Very Tanto=So much Troppo=Too much
So I think both can be translated to "so".
But by the examples that I saw:
If "so" describe a noun/verb -> the translation is tanto
It "so" describe an adjective or another adverb -> così
Translation by google translate + collins + daulingo:
I drink so much milk -> noun ("so" is an adjective) -> Bevo tanto latte
I work so much during the week -> verb -> Lavoro tanto in settimana.
she's so nice -> adjective -> È così simpatica
You drive so slowly -> adverb -> Guidi così lentamente
So in the sentence "It is so sweet" "so" describes the adjective -> therefore the translation is così.
I hope there is a native speaker who can confirm this :)
In terms of the direction of the accent, it really doesn't make much difference in Italian.
Either way will be accepted as correct, and most of the time will not even be noticed by a real Italian if you use the wrong angle.
In Italian, like most languages, the accent functions as a focal point in the word.
So, for example, by having the accent on the i in "così" it means you must stress that part of the sentence.
In Italian you must accentuate every letter in the word, but when there is an accent that word must be accentuated more than the rest.
If Cosi didn't have an accent you would pronounce is cOsI with equal weight on each letter / vowel.
Hope this helps.
Just an observation.
"this is so sweet" is just an observation.
If you would say "e' troppo dolce" then it could be argumentative OR observational. If someone said "Wow, this cake is soo nice" and you responded with "E' troppo dolce" then it would be seen as argumentative.
But if you simply stated "E troppo dolce" then you would just be making an observation.
The tone wouldn't really differentiate between the two like in a question vs statement sentence. E.g:
"Tu sei contento." vs "Tu sei contento?"
The one is stating, as an observation, that the person is content. While the latter is asking them a question and will only be noted through a change in your tone of voice.