Would it be possible to enable us to mark more difficult words and review them more often?
Please. I'm getting tired of duolingo asking me about words like Wohnung, verkaufen or Tochter over and over again.
With an adaptive system like Duolingo, inevitably some of your time will be spent teaching Duolingo what you do and don't find easy. Duolingo takes account of how long it takes you to answer the question, and it takes account of whether or not you "peek" - whether or not you hold the mouse over a word to see what it means. If you can answer the questions quickly and without peeking, the skills will decay more slowly. If you're doing more than a couple of lessons a day, then pretty quickly Duolingo will realise that you know those words, and you'll start seeing them a lot less often.
:) Yes, absolutely certain. There's some information here: https://support.duolingo.com/hc/en-us/articles/115000517426-Why-don-t-my-skills-stay-golden- - though I don't think those articles specifically mention timing. I've seen that mentioned many times on the forums, and I've seen a video with Luis and Severin talking about it.
Try a real 3rd party flashcard software like Memrise (includes difficult words, typing reviews in the L2 target language, disabling multiple-choice questions with Cooljingle's user script), AnkiSRS, FlashCards Deluxe, SuperMemo, etc.
Besides marking words automatically or manually as difficult on Memrise, there is also the IGNORE feature available.
I am not already experienced with the follow up upper-intermediate or advanced courses, not even for Portuguese BR.
There is a good community forum at: http://community.memrise.com
To pre-learn/re-learn/review the same DuoLingo vocabulary you would of course have to do the DuoLingo clone Memrise courses in parallel to your DuoLingo course here.
However, there are "Learn ... for Polyglots by EasyAcademy" courses available, where the typing variant often is about 120h long: https://www.memrise.com/course/250250/easy-academy
I know that the course descriptions contain the keywords: B1, B2, even C1.
But I am not actively learning with that course...not yet.