Can "I think often" work as well as "I think a lot"? "A lot" is actually an idiom.
"often" is translated as "frequentemente", they aren't exactly the same. On Duo it's better to use literal translations to save hearts =) You'll use "a lot" quite often, as well as "a little bit (of)" and other expressions...
No, there's a difference. 'Acho' translates as "I find" as in "I thought the music was terrible" = "I found the music terrible".
This sentence refers to the process of thinking about stuff, which requires the verb 'pensar'.
Yes, depending on the context and intonation. Also: "eu penso demais".