He's quite active under the comment thread for his castle:https://studio.bricklink.com/v2/build/afoldesigner_winner_model.page?id=62780
He's promised to share his MOC expansion instructions on Rebrickable once he's finished: His current page: https://rebrickable.com/users/Raziel_Regulus/mocs/
Finally, here's a recent comment that may shed some light on your questions about the bricks:
"I do what I can, but the thing with obtainablility is that it is a very relative term. The price for a specific piece can practically change drastically overnight, so I decided to focus more on Löwenstein Castle's partslist for reference. Currently the expansion has about 1500 pieces, about 1000 of which can be found in the original set. For the remaining parts I try to use ones not too specific, so that you can either get them from other LEGO themes or you already have them in your collection (the hut's red timber framing is an example for that). Another thing: The prices for some of the more useful elements for castles are partly ridiculous because of this "castle drought", like the rock pieces and arches, so I really would recommend considering to get a second Löwenstein if you plan to expand the set in any way really, unless you already have a big castle collection."