Kris has said there won't be a github for Marvin 3.
1. You can select multiple books when using OPDS. Yes you have to tap on them one at a time, but then you hit download and all those books you selected are downloaded. I haven't heard that Kris is working on changing that, but I am certainly not privy to all the inner-workings!
I think the more popular solution that he has said he is working towards is getting the Calibre plugin to work correctly. Then you would be able to simply ctrl-A your way to selecting all of them for transfer.
2. I personally asked about this functionality when we were beta testing Marvin Classic. The answer was that Marvin can't tell exactly what a book is unless/until it actually downloads the book. It can't tell if the book on your phone is the exact same as what you want to download. It would be nice if the "library-index-card" of information that OPDS supplies to Marvin would contain enough of that identification info, but apparently Title and Author aren't enough...could be a different version or something. Once Marvin has downloaded the book to see if it's a duplicate, it's kind of too late to save the time downloading it...