Regarding what to do for the rest of private beta...
Please note firstly that I'm referring to the questions here in this private beta until the graduation to the public beta. After this, the users are free to ask in the language they think is better to them.
Leave existing questions alone and don't demand future beta questions be in English. I don't think questions in Portuguese will scare off learners (like me), and contributions should be welcomed in whatever form users (who are volunteering their time) feel most comfortable. Many questions in Portuguese are advanced enough that learners would only need to ask them when they're advanced enough to try reading them.
Post new "duplicate" questions in English if you want, but just during private beta. On the other hand, if folks want to take initiative to make the site more friendly to learners who aren't ready for full immersion, there are some existing questions that learners will certainly want to ask (por que vs porque, et alpor que vs porque, et al; mil vs milharmil vs milhar; everything on pronúncia).
For those questions, I think it is better to make separate Q&As than to edit translations into existing ones that were conducted entirely in Portuguese. First, this lines up with our apparent consensus on answering questions in the language they were initially asked and allowing cross-language duplicates. Moreover, most answers will be substantially different when directed at an English speaker, so a simple translation of existing answers won't be sufficient. Finally, separate questions will be more easily found by learners.
As the OP argues, these may make the site more welcoming to learners. However, I don't think we should make creating English dupes an ongoing policy (long into beta), since we should have enough learners asking questions of their own soon enough.
(Just to clarify, I don't mean to suggest that every question in Portuguese needs a twin in English; nor that bilingual questions are problematic.)