Is it correct English? If it's, it should be okay.

Yup

Well, in that case "So much " will be translated as "tanto" its correct, and have the same meaning in the end. But considering Duoling tries to make you learn the mostly precisely translation, that is wrong, cuz in portuguse taht will be.

Eu os amo tanto instead of Eu os amo muito.

It's probably better to think of "Eu os amo muito" as Brazilian Portuguese because "Amo-os muito" is simply Portuguese. Interestingly, in the negative both EP and BP would use "(Eu) não os amo muito".

one would say "eu amo muito eles" in spoken language, but it's considered gramatically wrong.

"So much" would be "tanto" instead of "muito". Scroll up for more information.

Essa é a estrutura mais comum. O "A lot" geralmente vem depois do pronome objeto, que nesse caso é o "them".

Yes, in European Pt:

• Amo-os muito aos dois, mas não posso viver convosco.
• I love you both, but I can't live with you.

Editei

Eu os/as amo tanto.